lBR 


BERKELEY 


UNIVERSITY  OF 

AUFORN.A  J  REESE   LIBRARY 


SCIE 

LIBR/ 


UNIVERSITY  OF  CALIFORNIA 


Class 


m 


'  •.  4 


MICROSCOPICAL 

PHYSIOGRAPHY 


OF   THE 


OCK-MAKING  MINERALS: 


AN  AID  TO  THE 


MICROSCOPICAL  STUDY  OF  ROCKS. 


BY 

H.    EOSENBUSCH. 


TRANSLATED    AND   ABRIDGED    FOR    USE  IN  SCHOOLS   AND 

COLLEGES 


BY 
JOSEPH  P.   IDDINGS.  ' 

illustrates  fig  121  £ffi?ootr=ciits  ana  26  plates  of  $i)0tomtcrosrapt)0. 


NEW    YORK: 

JOHN    WILEY    &     SONS, 
15  ASTOR   PLACE. 

1888. 


COPYRIGHT,  1888, 
BY  JOHN  WILEY  &  SONS. 


DRUMMOND  &  ..ulj. 

Electrotypers, 

1  to  7  Hague  Street, 

New  York. 


FERRIS  BROS., 

Printers, 

326  Pearl  Street, 

New  York. 


e 

•- 


HARTH 

SCIENCES 
LIBRARY 


TRANSLATOR'S  PREFACE. 


IN  preparing  an  English  translation  and  abridgment  of  Professor 
Kosenbusch's  *'  Mikroskopische  Physiographic  der  petrographisch 
wichtigen  Mineralien,"  with  his  permission,  it  has  been  my  desire  to 
present  to  English-speaking  students  the  essential  features  of  this  val- 
uable work,  which  contains  all  that  is  necessary  for  an  accurate  and  com- 
plete determination  of  the  rock-making  minerals ;  hoping  that  in  so 
doing  I  may  not  only  meet  the  wants  of  those  who  take  up  unaided 
the  study  of  rocks,  but  may  assist  those  who  are  teaching  this  impor- 
tant branch  of  geology  by  providing  them  with  a  reference-book  con- 
taining the  diagnostic  characters  of  these  minerals.  It  is  also  hoped 
that  it  may  lead  to  a  more  general  interest  in  and  a  more  accurate 
knowledge  of  microscopical  petrography  in  this  country,  and  may 
increase  the  number  of  those  who,  by  exact  and  patient  study,  shall 
add  to  the  store  of  established  facts,  and  .thus  advance  the  science  of 
lithology. 

In  abridging  the  book,  I  have  endeavored  to  retain  all  that 
appeared  to  be  essential  to  a  fair,  general  comprehension  of  the  sub- 
ject, omitting  what  seemed  to  be  refinements  beyond  the  need  of  the 
average  student,  and  for  which  the  advanced  student  is  referred  to  the 
original  work.  Thus  most  of  the  historical  portions  have  been  omit- 
ted, as  well  as  the  elaborate  treatment  of  the  optical  anomalies  of  cer- 
tain minerals,  and  many  notes  on  European  localities;  while  a  number 
of  notes  on  American  occurrences  have  been  inserted. 

In  two  instances  I  have  taken  the  liberty  of  departing  from  the 
original  in  the  use  of  names,  the  grounds  for  which  Professor  Rosen- 
busch  will  undoubtedly  appreciate.  The  term  Sp7i<irokrystal  has 
been  rendered  spheruliie,  as  the  latter  has  become  well  established  in 
English  petrographical  literature,  and  is  not  open  to  the  objection 
which  might  be  raised  against  the  new  term.  Liparite  has  been  ren- 
dered rhyolite,  also  for  the  reason  that  it  is  in  such  general  use  in  this 
country  and  in  England  that  to  supplant  it  would  lead  to  great  coi 
fusion.  With  these  exceptions  I  trust  Professor  Rosenbusch  may  find 


vi  VORWORT. 

Bedeutung  gerade  der  optischen  Eigenschaften  fiir  die  Erkennung 
der  Mineralien  unter  dem  Mikroskope  und  die  oft  geraachte  Er- 
fahrung,  wie  sehr  dieselben  von  den  Studirenden  zu  ihrem  eigenen 
grossten  Schaden  vernachlassigt  werden.  Allenthalben,  wo  es  nothig 
schien,  babe  icb  die  besprocbenen  Verhaltnisse  durch  scbematiscbe 
Zeichnungen  zu  erlautern  gesucht;  dieselben  sollen  nur  die  An- 
schaunng  erleicbtern  und  machen  auf  strenge  Winkelgenauigkeit 
keinen  Anspruch.  --  Die  ganze  Anlage  und  der  Zweck  des  Bucbes, 
welcbes  ja  kein  Lehrbuch  der  Mineral- Optik  sein  soil,  diirften  es 
wobl  hinreichend  erklaren,  dass  nicbt  eine  strengere  Form  fiir  die 
Besprechung  dieser  Yerhaltnisse  gewablt  wurde.  Aebnlicbe  piida- 
gogische  Erwagungen  leiteten  mich  aucb,  wenn  ich  z.  B.  die  Er- 
lauterung  der  optischen  Erscbeinungen  in  diinnen,  doppeltbrechenden 
Minerallamellen  irn  polarisirten  Licbte  der  Erkliirung  der  gleichen 
Phanomene  in  dickeren  Krystallplatten  vorausgeben  liess,  obwobl 
ja  die  im  ersten  Fall  auftretenden  Farben  nur  der  centrale  Theil 
des  Bildes  sind,  welcbes  wir  im  zweiten  Fall  erhalten. 

Aus  der  reicblicben  Benutzung  fremder  Arbeiten  wird  mir  urn 
so  weniger  ein  Yorwurf  erwacbsen  konnen,  als  ich  die  Resultate 
derselben,  wo  es  mir  nur  irgend  moglich  war,  stets  an  eigenhandig 
gefertigten  Priiparaten  gepriift  babe.  Die  wenigen  Fiille,  wo  das 
nicbt  geschehen  konnte,  wird  man  beim  Lesen  des  Textes  leicht 
herausfinden. 

Ich  war  lange  schwankend,  ob  nicht  aucb  ein  Abscbnitt  iiber 
die  Tecbnik  des  Mikroskopes  hatte  aufgenommen  werden  sollen  ; 
scbliesslicb  war  der  Umstand  entscheidend,  dass  ein  solcher,  wenn 
er  nutzbringend  sein  sollte,  das  Buch  unverhaltnissmassig  vergrossert 
und  also  vertheuert  baben  wiirde.  Hierfiir  muss  demnach  auf  die 
einscblagigen  Werke,  besonders  das  von  HARTING,  verwiesen  werden. 

ISTach  meinen  Erfabrungen  wird  der  Werth  mikroskopiscber  Be- 
schreibungen  wesentlich  durch  bildliche  Darstellung  erhoht ;  es  war 
daher  mein  Bestreben,  diese  in  moglicbster  Reichhaltigkeit,  theils 
als  Holzschnitte  im  Text,  theils  in  den  Farbentafeln  zu  geben.  Dass 
bei  den  ersteren  manche  fremde  Zeichnung  init  Angabe  der  Quelle 
benutzt  wurde,  bedarf  wohl  keiner  Entschuldigung.  Bei  Anfertigung 
der  Tafeln,  auf  denen  sich  nur  eigene  Zeichnungen  finden,  babe  ich 
mit  Fernhaltung  Alles  dessen,  was  man  Schematisirung  derselben 
nennen  konnte,  stets  eine  absolut  objective  Wiedergabe  des  mikro- 
skopischen  Bildes  angestrebt.  -  -  Dass  auf  dei?  Farbentafeln  die 
Mineralien  der  spiiteren  Sjsteme  gegen iiber  den  amorpben  und  re- 
gularen  etwas  stiefmiitterlicb  behandelt  worden  sind,  hat  seinen 


VORWORT.  vii 

Grund  darin,  dass  urspriinglich  mehr  Tafeln  in  Aussicht  genommen 
waren.  Doch  beliefen  sich  die  Kosten  fiir  Anferti^ung  derselhen 
so  hoch,  dass  ihre  Zabl  auf  10  beschrankt  werden"  musste,  wenn 
nicht  allzu  weit  iiber  das  billige  Maass  hinausgehende  Anforderungen 
an  die  dankbar  anzuerkennende  Opferwilligkeit  des  Herrn  Verlegers 
gestellt  werden  sollten.  ~-  Von  den  Zeichriungen  solcher  mikro- 
skopischer  Verhaltnisse,  die  schon  in  leicbt  zuganglichen  Special- 
arbeiten  eine  graphische  Darstellung  gefunden  batten,  konnte  Ab- 
stand  genommen  werden.  Ferner  wurde  darauf  gesehen,  solches 
Material  als  Object  zu  den  Zeicbnungen  zu  wahlen.  welches  un- 
scliwer  fiir  Jeden  zu  beschaffen  ist,  damit  der  Lernende  an  selbst- 
angefertigten  Praparaten  nach  Anleitung  des  Buches  seine  Beob- 
achtimgen  und  Studien  macben  konne.  Denn  das  muss  man  nicht 
vergessen :  mit  dem  blossen  Lesen  und  Studiren  ist  es  nicht  gethan ; 
-  wer  mikroskopische  Mineralogie  lernen  will,  muss  an  den  Schleif- 
tisch  und  an  das  Mikroskop. 

Die  genaue  und  gewissenhafte  Angabe  der  Literatur  bei  jedem 
Gegenstande,  sowie  die  Zusammenstellung  derselben  am  Schlusse 
des  Buches,  diirfte  auch  dem  Fachtnann  nicht  ganz  unwillkommen 
sein  und  ist  besonders  darauf  berechnet,  dem  Anfanger  Gelegenheit 
zu  geben,  sich  in  die  historische  Entwicklung  der  Wissenschaft  ein- 
zuleben.  Eine  eingehende  Kenntniss  der  Geschichte  der  Wissen- 
schaft scheint  mir  durchaus  nothwendig,  um  den  organischen  Zu- 
sammenhang  des  Individuums  mit  der  Gesammtheit  herzustellen, 
durch  welchen  allein  die  fordernde  Einheit  und  das  klare  Bewusst- 
sein  der  anznstrebenden  Ziele  in  die  wissenschaftliche  Entwicklung 
kommt.  Ferner  aber  kann  nur  durch  die  historische  Kenntniss 
seiner  Wissenschaft  jedem  Studirenden  das  Seiende  als  ein  Gewor- 
denes  erscheinen  und  ihn  erkennen  lassen,  wie 

Alles  sich  zum  Ganzen  webt, 

Eins  in  dem  Andern  wirkt  und  lebt. 

Sollte  hie  und  da  eine  nennenswerthe  Arbeit  unerwahnt  ge- 
blieben  sein,  so  bitte  ich  das  im  Hinblick  darauf  zu  entsclmldigen, 
dass  ja  dem  Einzelnen  nicht  alle  Biicher  und  alle  Zeitschriften  zu- 
giinglich.  sind.  Fiir  Belehrung  und  Unterstiitzung  in  dieser  Rich- 
tung  wiirde  ich  in  ganz  besonderem  Grade  dankbar  sein. 

Im  Ganzen  und  Grossen  glaube  ich,  gestiitzt  auf  die  Erfahr- 
ungen,  die  ich  im  akademischen  Vortrage  des  Gegenstandes  dieses 
Buches  zu  sammelti  Gelegenheit  hatte,  einen  nicht  durchaus  falschen 
Weg  eingeschlagen  zu  haben,  bescheide  mich  aber  gern  gegeniiber 


vni  VORWORT. 

dera  Urtbeil  erfabrenerer  Forscher,  deren  sachliche  Kritik  mir  in 
hohem  Grade  willkommen  sein  wird. 

Trotz  aller  angewandten  Sorgfalt  sind  im  Text  einige  Druck- 
fehler  stehen  geblieben,  deren  Yerzeichniss  angebeftet  1st  und  die 
ich  zu  corrigiren  bitte. 

Scbliesslicb  fiihle  icb  micb  gedrungen,  meiuem  Freunde,  Herrn 
Professor  H.  FISCHER  in  Freiburg,  den  aufriclitigsten  Dank  fur  die 
unerraiidliche  Bereitwiiligkeit  auszusprechen,  womit  er  durcb  die 
Erlaubniss  zur  Benutzung  des  akademiscben  Cabinets,  seiner  Privat- 
bibliothek  und  seiner  reicben  Sammlung  mikroskopiscber  Praparate 
meine  Arbeit  freundlicbst  gefordert  bat. 

Freiburg  i.  B.  im  Mai  1873. 

H.    ROSENBUSCH. 


VORWORT  ZUR  ZWEITEN  AUFLAGE. 


Die  gewaltigen  Fortschritte,  welche  die  mikroskopische  Mineral- 
diagnose  seit  dem  Jahre  1873  gemacht  hat,  bedingten  eine  voll- 
kommene  Umarbeitung  der  ersten  Auflage  dieses  Buches.  Ander- 
weitige  Beschaftigungen  und  eine  angestrengte  Lehrthiitigkeit  ver- 
zogerten  die  Erf iillung  dieser  Aufgabe  fast  iiber  Gebiihr. 

Ob  es  mir  gelungen  ist,  eine  dem  heutigen  Standpunkt  unserer 
Wissenschaft  entsprechende  Darstellung  des  Gegenstandes  zu  geben, 
.  muss  ich  der  Beurtheilung  berufener  Fachgenossen  iiberlassen.  Die 
hohe  Vervollkommnung  der  Methoden  und  Instrumente,  sowie  eine 
gewisse  Yerschiebung  der  Ziele  und  Gesichtspunkte  bei  mikrosko- 
pischen  Mineraluntersuchungen,  welche  in  dem  letzten  Decennium 
sich  vollzogen  hat,  bedingten  rnanche  durchgreifende  Aenderung  in 
dem  Plane,  nach  welchem  die  erste  Auflage  dieses  Buchs  be- 
arbeitet  war.  Das  rein  Descriptive  in  derselbeu  musste  auf  das 
unumganglich  nothwendige  Maass  beschriinkt,  das  Hauptgewicht  auf 
die  Anleitung  zu  einer  moglichst  exacten  mikroskopischen  Bestimmung 
der  Mineralien  gelegt  werden.  Immerhin  durfte  und  sollte  dieses 
Buch  kein  Lehrbuch  der  Krystalloptik  werden,  es  sollte  ein  Hiilfs- 
buch  bei  petrogr  aph  i  sch  en  Untersuchungen  bleiben.  Das  be- 
dingte  in  manchen  Punkten  eine  Abweichung  von  den  strengen 
Methoden  der  Optik,  iiber  deren  Berechtigung  man  verschiedener 
Ansicht  sein  kann  und  wird.  Ich  habe  mich  bei  der  Behandlung 
des  Gegenstandes  durch  die  Erfahrungen  leiten  lassen,  welche  ich 
in  dem  Zusammenarbeiten  mit  lieben  Schiilern  seit  nunmehr  16 
Jahren  habe  sammeln  konnen. 

Die  auf  mikroskopische  Mineralogie  beziigliche  Literatur  ist 
eine  so  zahlreiche  geworden  und  eine  ihrem  inneren  Werthe  nach 
so  verschiedene,  dass  eine  voile  Beriicksichtigung  derselben,  wie 
im  Jahre  1873,  nicht  moglich  war.  Ich  habe  mich  jedoch  bestrebt, 
dieselbe  in  solcher  Yollstandigkeit  bei  jedem  Gegenstande  zu  geben, 
dass  der  Leser  ohne  Schwierigkeit  die  historische  Entwicklung  der 
Erkenntniss  desselben  verfolgen  kann.  Yor  jedem  Capitel  steht  das 
allgemein  Wichtige  aus  derselben,  in  Fussnoten  das  nur  oder  mehr 


X  VORWORT  ZUR  ZWEITEN  AUFLAGE. 

einzelfallig  Bedeutsame.  Nicht  aufgefiihrt  wurden  die  Lehrbiichet 
von  F.  FOUQUE  und  A.  MICHEL-LEVY,  von  ERN.  MALLARD,  G.  TSCHER- 
MAK  und  J.  YERDET,  die  ich  ausgiebig  benutzt  habe.  Ihr  lioher 
Werth  bedarf  nicht  meiner  besonderen  Anerkentmng.  —  Einen  nach 
Moglichkeit  vollstandigen  Literaturnacbweis  habe  ich  dem  Buche 
auf  vielseitig  mir  ausgesprochenen  Wunsch  arigehangt;  ich  folge 
darin  dem  Rathe  von  lieben  Fachgenossen,  auf  deren  Urtheil  ich 
schr  grosses  Gevvicht  lege,  fast  gegen  meinen  WiJlen. 

An  die  Stelle  der  10  chromolithographischen  Tafeln  der  ersten 
Auflage  siud  26  Tafeln  in  Lichtdruck  getreten ;  dieselben  machten 
viele  lange  Beschreibungen  entbehrlich.  Herr  Professor  COHEN  in 
Greifswald  gestattete  frenndlich  die  weitestgehende  Benutzung  sei- 
ner schonen  Mikrophotographieen.  Auch  znr  Yervollstandigung  des 
Literaturnachweises  half  er  mir  in  bereitwilligster  Weise.  Ebenso 
erfreute  ich  mich  der  liebenswiirdigen  Unterstiitzung  und  des  be- 
wahrten  Rathes  des  Herrn  Professor  KLEIN  in  Gottingen.  Beiden 
lieben  Freunden  danke  ich  auch  an  dieser  Stelle  nochmals  lierzlich. 

Die  Herstelhing  der  NEWTON 'schen  Farbenskala  war  nicht  ohne 
Schwierigkeit ;  es  erwies  sich  uurnoglich,  dieselbe  mit  andern,  als 
mit  Anilinfarben  auszufiihren.  Man  wolle  sie  also  thunlichst  vor 
Sonnenlicht  schiitzen.  Dieselbe  wird  hoffentlich  die  benutzung  der 
Interferenz-Erscheinungen  zur  Mineralbestimrnung  dem  Anfiinger 
wesentlich  erleichtern. 

Die  Beschreibnng  des  neuen  FuESs'schen  Mikroskops,  welches 
mir  erst  vor  wenigen  Tagen  bekannt  wurde,  glaubte  ich  in  einem 
Nachtrage  geben  zu  sollen. 

Der  Herr  Yerleger  ist  mit  solcher  fretmdlichen  Bereitwillig- 
keit  auf  jeden  meiner  Yorschliige  zur  Ausriistung  dieses  Buches, 
ohne  Rucksicht  auf  die  dadurch  erwachsenden  Kosten  und  Miihen 
eingegangen,  dass  ich  mich  ihm  in  hohem  Grade  verpflichtet  fiihle. 

Allen  meinen  lieben  Schulern  und  Freunden  endlich,  nah  urid 
fern,  die  hier  die  Friichte  ihres  Fleisses  verwerthet  linden,  moge 
dieses  Buch  eine  Erin nerung  sein  an  die  frohen  Tage  gemeinschaft- 
licher  Arbeit  und  Anregung  ! 

Einige  sinnstorende  Druckfehler  wolle  man  vor  der  Benutzung 
des  Buches  corrigiren. 

Heidelberg,  September  1885. 

H.    ROSENBUSCH. 


TABLE  OF  CONTENTS. 


PAGE 

DEFINITION,    ...  •  •'..'....  1 

PREPARATION  OP  MATERIAL  FOR  STUDY,     .  „  1-8 


GENERAL  PAKT. 

MORPHOLOGICAL    CHARACTERS. 

I.  CRYSTALS  AND  CRYSTAL  SECTIONS, .,  4-6 

II.  NORMAL  AND  ABNORMAL  CRYSTALLIZATION, 6-20 

(a)  The  External  Form.     Forms  of  growth;  globulites,  cumulites,  mar- 

garites,  longulites,  crystallites ;  trichites,  spherulites  (Sphdrokry- 
stal(e),  skeleton  crystals,  microlites ;  mechanical  deformation ; 
chemical  deformation,  .  ^  '  ;  ..  .  !.  v  *  .  .  6-12 

(b)  The  Internal  Structure,  or  the  Homogeneity.     Shelly  structure  from  in- 

termittent growth  and  from  isomorphous  lamination;  inclusions; 
gas  inclusions,  fluid  inclusions  and  their  bubbles;  water,  salt  solu- 
tions and  liquid  carbon  dioxide,  two  unmiscible  fluids ;  crystalliza- 
tions in  fluid  inclusions  ;  glass  inclusions,  stone  inclusions ;  in- 
dividualized inclusions ;  arrangement  of  inclusions,  .  .  12-19 

(c)  Twins, .19 

(d)  Aggregates, .        .20 


PHYSICAL  PROPERTIES. 

I.  PHENOMENA  OF  COHESION.     Cleavage,         .     .  ^       .        .        .        .       21-23 
II.  OPTICAL  PROPERTIES,     .        •        •        .,*.•„      *k  /  ....  .....,,,      .        .       23-90 

(a)  Refraction  and  Index  of  Refraction  in  Isotropic  'Media.     Law  of  refrac- 

tion, limiting  angle,  total  reflection,  relief,  illumination  and  rough 
surface  of  mineral  sections,  determination  of  the  index  of  refrac- 
tion, .'  ."  .'."..  .  .  .  .  .  .  23-31 

(b)  Double  Refraction  in  Anisotropic  Media,  with  one  and  with  two  optic 

axes,  optical  character,  optic  axes,  principal  indices  of  refraction, 
bisectrices  ;  optical  characteristics  of  the  three  crystal  systems  with- 
out a  principal  axis,  dispersion  ;  influence  of  temperature  and  pres- 
sure on  the  double  refraction  ;  partial  pressure  and  unequal  eleva- 
tion of  temperature,  .  .  .  .  .  .  .  .  .  31-46 

(c)  Investigation  of  Minerals  in  Parallel  Polarized  Light.     Polarization  in- 

struments, tourmaline  tongs,  cosine  law,  nicol  prisms,  polarizing 
microscope  ;  iso tropic  mineral  plates  in  parallel  polarized  light, 
thin  plates  of  doubly  refracting  minerals  in  parallel  polarized  light ; 
deduction  of  the  law  for  the  interference  ray,  Newton's  scale  of 


Xll  TABLE    OF    CONTENTS. 

PAGE 

colors ;  behavior  of  doubly  refracting  plates  cut  at  right  angles  to 
an  optic  axis  in  parallel  polarized  light ;  behavior  of  several  such 
plates  lying  upon  one  another  in  polarized  light ;  plates  of  aniso- 
tropic  twinned  crystals  in  polarized  light ;  stauroscopic  methods  for 
the  determination  of  the  direction  of  extinction  in  doubly  refract- 
ing plates  ;  determination  of  the  relative  values  of  both  axes  of 
elasticity  in  doubly  refracting  plates  ;  determination  of  the  indices 
of  refraction  in  doubly  refracting  plates  ;  table  of  the  mean  indices 
of  refraction  of  the  rock-making  minerals,  ....  46-66 

(d)  Investigation  of  Minerals  in  Convergent  Polarized  Light.   The  microscope 

as  a  Norremberg's  polarization  instrument ;  interference  phenom- 
ena of  uniaxial  plates,  cut  perpendicular  to  an  axis,  in  conver- 
gent polarized  light ;  uniaxial  plates  parallel  to  the  axis  in  conver- 
gent polarized  light ;  plates  of  optically  biaxial  crystals  perpendic- 
ular to  an  axis  ;  such  plates  perpendicular  to  a  bisectrix  ;  dispersion 
phenomena  of  orthorhombic,  monoclinic  and  triclinic  minerals ; 
determination  of  the  optical  character  of  uniaxial  and  biaxial  plates 
in  convergent  light  by  means  of  the  quarter  undulation  mica  plate 
and  the  quartz  wedge,  .  .  .  .  .  .  .  .  67-83 

(e)  Color  of  Minerals.     Color  and  pigment ;  pleochroism  of  uniaxial  and 

biaxial  minerals,  microscopic  determination  of  pleochroism  ;  pleo- 
chroism of  biaxial  minerals;  pleochroic  halos,  artificial  produc- 
tion of  pleochroism "•'..  .  83-87 

(/)  Aggregates.      Optical   behavior;   aggregate  polarization.      Spherical 

aggregates;  spherulites,  axiolites,      .         .         ...       ...        88-90 

III.  CHEMICAL  PROPERTIES,          .        .        .        .        *        .        .        .        .      91-114 

(a)  Chemical  Investigation  on  Thin  Sections.     Treatment  with  reagents, 

evolution  of  gas,  solution,  gelatinization  and  tingeing;  etching ; 
heating  to  redness,  .  ., 93-98 

(b)  Micro  chemical  Investigation  of  Loose  Grains.  Preparation  of  the  material, 

separation  according  to  specific  gravity,  separating  fluids  of  Thoulet, 
Klein,  Rohrbach,  Braun,  and  separating  apparatus  of  Harada, 
Brogger ;  separating  funnel,  indicators ;  determination  of  the 
specific  gravity  of  separating  fluids  and  grains  suspended  therein  ; 
separation  of  a  mixture  of  grains  by  means  of  an  electro-magnet  ; 
separation  of  minerals  by  chemical  ways ;  determination  of  the 
specific  gravity  of  isolated  powder  ;  table  of  specific  gravities  of 
rock-making  minerals  ;  determination  of  the  hardness  of  isolated 
powder;  micro-chemical  reactions,  ...  .  .  .  98-114 


SPECIAL  PART. 

CLASSIFICATION,  .  .  .'  ,  .  .  •  •  .  •  .  .  115-120 
AMORPHOUS  MINERALS.  Opal,  carbonaceous  matter,  .  „  £  .  •  121-123 
MINERALS  OF  THE  ISOMETRIC  SYSTEM,  Pyrite,  magnetite,  chromite,  spinel  group, 

fluorite,  garnet  group,  leucite,  sodalite  group,  analcite,  perofskite,  .  124-143 
MINERALS  OF  THE  TETRAGONAL  SYSTEM.  Rutile,  anatase,  cassiterite,  zircon, 

scapolite  group,  vesuvianite,  melilite, 144-160 

MINERALS  OF  THE  HEXAGONAL  SYSTEM.  Graphite,  pyrrhotite,  hematite, 

ilmenite,  corundum,  brucite,  quartz,  chalcedony,  tridymite,  calcite,  dolo- 


TABLE    OF   CONTENTS.  xiii 

PAGE 

mite,  magnesite,  apatite,  nepheline  and  elseolite,  cancrinite,  tourmaline, 
eudialite,  chlorite  group, •  .  161-188 

MINERALS  OF  THE  ORTHORHOMBIC  SYSTEM.  Brookite,  pseudobrookite,  arago- 
nite,  anhydrite,  andalusite,  sillimanite,  topaz,  staurolite,  group  of  ortho- 
rhombic  pyroxenes,  bastite,  group  of  the  orthorhombic  amphiboles,  olivine, 
cordierite,  zoisite,  talc,  natrolite, 189-225 

MINERALS  OF  THE  MONOCLINIC  SYSTEM.  Gypsum,  wollastonite,  group  of  mono- 
clinic  pyroxenes,  group  of  monocli-nic  amphiboles,  mica  group,  ottrelite 
group,  epidote,  allanite,  titanite,  nionoclinic  potash  feldspars,  .  .  226-286 

MINERALS  OF  THE  TRICLINIC  SYSTEM.  Microcline,  group  of  plagioclases,  dis- 
thene,  axinite,  cossyrite, 287-316 

HOMOGENEOUS  AGGREGATES.     Serpentine,  delessite,  kaolin,        .        .        .    317-320 

INDEX, •  .          ...  321 

EXPLANATION  OF  PLATES,  I-XXVL,  ...      .   .          ,327 


ABBREVIATIONS. 


N.  J.  B.  —  Jahrbuch,  or  Neues  Jahrbuch  fur  Mineralogie,  Geologic  und  Palaonto- 

logie.     Stuttgart. 

Z.  D.  G.  G.  —  Zeitschrift  der  deutschen  geologischen  Gesellschaft.     Berlin. 
P.  A.  =  POGGENDORF'S  Annalen  fiir  Physik  und  Chemie.     Leipzig. 

A.  M.  =  Annales  des  mines.     Paris. 

S.  W.  A.  =  Sitzungsberichte  der  K.  K.  Akademie  der  Wissenschaften  zu  Wien. 

B.  M.  =  Monatsbericht  der  K.  Akademie  der  Wissenschaften  zu  Berlin. 

S.  B.  A  =  Sitzungsberichte  der  K.  Akademie  der  Wissenschaften  zu  Berlin. 

S.  M.  A.  =  Sitzungsberichte  der  K.  bayrischen  Akademie  der  Wissenschaften  zu 

Mii  lichen. 

T.  M.  M.  Mineralogische  Mittheilungen  ges.  von  G.  TSCHERMAK.     Wien. 
T.  M.  P.  M.  =  Mineralogische    und    petrographische    Mittheilungen    ges.   von   G. 

TSCHERMAK.     Wien. 
A.  Ch.  Ph.  =  Annales  de  Chimie  et  de  Physique.     Paris. 

A.  Ch.  Pharm.  =  Annalen  der  Chemie  und  Pharmacie.     Leipzig. 

Z.  X.  —  Zeitschrift  fiir  Krystallographie  und  Mineralogie,  her.   von    P.  GROTH. 
Leipzig. 

C.  R.  —  Comptes  rendus  hebdomadaires  de  1'Acadeinie  franyaise.     Paris. 

F.  K.  =  Foldtani  Kozlony.     Budapest. 

Min.  Mag.  =  Mineralogical  Magazine.     London. 

Geol.  Mag.  —  Geological  Magazine,  etc.     London. 

Q.  J.  G.  S.  =  Quarterly  Journal  of  the  Geological  Society.     London. 

B.  S.  M.  and  Bull.  Soc.  Min.  Fr.  =  Bulletin  de  la  Societe  mineralogique  de  France. 

Paris. 
Bull.  Soc.  geol.  Fr  —  Bulletin  de  la  Societe  geologique  de  France.     Paris. 

G.  F.  i   Stockh.  Forhdl.  =  Geologiska  Foreningens  i  Stockholm  Forhandlingar. 

Stockholm. 


MICROSCOPICAL   PHYSIOGRAPHY 

OF  THE 

ROCK-MAKING  MINERALS 


THE  Microscopical  Physiography  of  rock-making  minerals  describes 
the  characteristics  by  which  these  minerals  may  be  determined  in  thin 
section  or  in  grains  by  transmitted  light  under  the  microscope. 

It  may  be  divided  into  two  parts :  a  general part^  in  which  the  three 
great  classes  of  mineral  characteristics,  the  morphological,  physical, 
and  chemical,  are  applied  to  microscopical  diagnosis  ;  and  a  special  part, 
which  contains  the  particular  description  of  each  mineral  species  as  it 
appears  under  the  microscope. 

PREPARATION  OF  MATERIAL. 

Literature. 

J.  G.  und  L.  G.  BORNEMANN,  Ueber  eine  Schleifmaschine  zur  Herstellung  mikro- 

skopischer  Gesteinsdiinnschliffe.     Z.  D.  G.  G.  1873.  XXV.  367-374. 
H.  CLIFTON  SORBY,  On  the  microscopical  character  of  sands  and  clays.   Monthly 

Microscop.  Journ.  1877.  February  7. 

G.  STEINMANN,  Eine  verbesserte  Steinschneidemaschine.      N.  J.  B.  1882.  II.  46-54. 
J.  THOULET,  Note  sur  un  nouveau  precede  d'etude  an  microscope  des  mineraux  en 

grains  tres-fins.     Bull.  Soc.  Miner.  Fr.  1879.  II.  188. 
H.  VOGELSANG,   Philosophic  der  Geologie  und  mikroskopische   Gesteinsstudien. 

Bonn  1867,  225-228. 
F.  ZIRKEL,  Mikroskopische  Gesteinsstudien.     S.  W,  A.  1863.  XL VII.  227-229. 

The  microscopical  investigation  of  minerals  or  mineral  aggregates 
is  carried  on  by  observing  them  by  transmitted  light,  either  in  thin 
plates  with  parallel  faces,  called  thin  sections,  or  in  the  form  of  powders. 
In  general,  for  optical  diagnosis  thin  sections  are  the  most  convenient ; 
while  for  microchemical  determination  a  thin  section  is  to  be  pre- 
ferred in  some  cases,  mineral  powder  in  others. 

The  manner  of  preparing  a  thin  section  may  be  modified  in  many 
ways.  In  cases  where  it  is  desired  to  prepare  thin  sections  in  a  par- 
ticular direction  through  a  mineral  or  rock,  thin  plates  are  cut  by  means 


2  PHT8IOOEAPHY  OF  THE  ROCK-MAKING  MINERALS. 

of  a  stone-cutting  machine,  using  a  metal  disk  set  with  diamond-dust 
or  emery.  If  direction  is  of  no  consequence,  it  is  better  to  chip  with 
a  hammer  thin  splinters  or  flakes  from  the  material  to  be  studied. 
These  chips  should  not  be  less  than  half  an  inch  in  diameter,  and  with- 
out cracks  or  flaws.  One  side  of  the  chip  or  of  the  thin  plate  is 
ground  plane  and  smooth.  The  grindstone  may  be  a  fixed  cast-iron 
plate,  emery-stone,  sandstone,  or  whetstone,  with  or  without  emery ; 
but  it  is  more  convenient  to  use  a  small  grinding-machine  having  a 
vertical  axis,  on  which  may  be  screwed  horizontal  grindstones  of  differ- 
ent coarseness.  Having  prepared  a  plane  surface  which  should  extend 
across  the  whole  chip,  it  is  then  polished,  carefully  washed  with  a  stiff 
brush,  and  dried. 

The  chip  is  then  fastened  or  cemented  to  a  thick  object-glass  by 
means  of  Canada  balsam.  The  object-glass  should  not  be  too  thin,  as 
it  will  bend  under  the  pressure  of  the  fingers,  and  the  edges  of  the 
rock  section  will  grind  away  faster  than  the  middle,  producing  a  len- 
ticular-shaped section.  The  Canada  balsam  may  be  used  in  a  viscous 
form,  being  handled  with  a  glass  rod,  or  after  it  has  been  hardened  by 
evaporation.  An  excellent  cement,  which  is  to  be  preferred  to  Canada 
balsam,  is  obtained  by  slowly  melting  together  a  mixture  of  16  parts 
by  weight  of  viscous  Canada  balsam  and  50  parts  of  shellac,  and  keep- 
ing them  heated  for  some  time.  The  mass,  before  it  completely  cools, 
may  be  drawn  out  in  strings  and  rolled  between  the  hands  into  con- 
venient rods  about  1  cm.  thick  and  20-30  cm.  long. 

The  chip  is  cemented  in  the  following  manner :  Spread  over  the 
thoroughly  cleaned  and  gently  heated  object-glass  a  continuous  coat  of 
cement, — which  should  not  be  too  thin, — at  the  same  time  heating  the 
chip  with  the  polished  side  up  to  drive  off  any  moisture,  and  then  lay  it 
with  the  polished  side  on  the  balsam.  The  object-glass  is  then  gradu- 
ally heated  till  the  balsam  loses  most  of  its  turpentine,  care  being  taken 
that  no  bubbles  form.  On  cooling,  press  the  chip  in  place.  When  the 
whole  is  thoroughly  cold,  the  exposed  surface  is  ground  down  as  be- 
fore. The  grindstone  is  used  as  long  as  possible  with  safety  to  the 
section,  which  in  most  cases  becomes  nearly  or  quite  transparent.  It 
is  then  ground  on  the  whetstone  until  it  is  completely  transparent.  In 
place  of  the  whetstone  a  smooth  glass  plate  may  be  used  with  water, 
together  with  the  finest  possible  emery-dust  floated  off  from  previously 
used  emery. 

The  thinness  required  in  a  particular  case  depends  on  the  object  of 
the  study,  and  may  be  determined  by  examining  the  section  from  time 
to  time  with  the  microscope  during  the  final  process  of  grinding,  the 


PREPARATION  OF  MATERIAL.  3 

section  being  first  moistened  with  water.  When  the  grinding  is  fin- 
isked  the  superfluous  balsam  around  the  rock  section  is  removed  with  a 
heated  knife-blade,  and  the  section  is  thoroughly  washed  with  a  stiff 
brush  and  alcohol,  rinsed  quickly  in  water,  dried  with  a  linen  cloth, 
and  then  brushed. 

A  new  object-glass  having  been  thoroughly  cleaned,  a  drop  of  Canada 
balsam  is  put  upon  it  and  gently  warmed  so  that  it  spreads  slightly.  The 
old  object-glass  is  then  gradually  heated  till  the  balsam  is  melted,  when 
the  rock  section  is  slid  on  to  the  new  glass  and  the  balsam  heated  so 
that  the  section  adheres  to  it.  Another  drop  of  balsam  is  put  upon 
the  section,  and  the  thinnest  possible  glass  cover  placed  over  it.  The 
whole  is  gradually  heated  and  the  glass  cover  pressed  closely  down. 
The  superfluous  balsam  is  removed  by  a  warm  knife-blade  and  alcohol, 
rinsing  thoroughly  with  water. 

In  many  cases,  where  the  material  to  be  studied  is  in  the  condition 
of  small  particles  (sand,  volcanic  ashes,  etc.),  or  where  its  composition 
does  not  permit  of  the  preparation  of  a  thin  section  (clay,  mud,  etc.), 
or,  finally,  where  the  mineral  elements  of  a  rock  have  been  separated 
for  individual  study,  the  choice  of  methods  for  the  preparation  of 
material  for  observation  depends  on  whether  the  outward  form  or  the 
internal  structure  is  the  special  object  of  investigation.  In  either  case 
it  is  advisable  to  use  powder  of  very  nearly  equal  grain.  This  is  easily 
obtained  by  freeing  the  entire  powder  of  dust  by  repeated  washing  in 
water  and  decantation,  and  then  passing  it  through  a  graduated  series 
of  sieves. 

If  the  outward  form  of  the  powder  is  to  be  studied,  it  is  best  to 
place  it  in  a  fluid  whose  index  of  refraction  is  considerably  lower  than 
that  of  the  solid  body.  Water  is  therefore  used,  care  being  taken  not 
to  suspend  too  much  powder  in  the  water.  The  whole  is  covered  with 
a  glass  to  give  it  a  plane  surface. 

On  the  other  hand,  if  the  internal  structure  is  to  be  studied,  one 
must  avoid  the  total  reflection  from  the  surface  of  the  solid  which 
occurs  with  water,  and  employ  a  medium  whose  index  of  refraction  is 
as  near  that  of  the  solid  body  as  possible.  The  substances  most  fre- 
quently used  are  glycerine  (n  —  1.46),  almond  oil  (n  =  1.47),  cassia  oil 
(n  =  1.606),  or  a  concentrated  solution  of  iodide  of  potassium  and  mer- 
cury (n  =  1.733).  Canada  balsam  (n  =  1.54)  is  not  so  satisfactory,  for 
the  powder  is  apt  to  crowd  together  when  the  balsam  is  melted.  This 
may  be  avoided  by  spreading  grains  of  powder  over  a  thin  film  of  cold 
balsam,  heating  slightly  so  the  grains  will  adhere,  and  covering  all  with 
balsam  dissolved  in  ether  or  chloroform,  and  with  a  glass  cover. 


GENERAL  PART. 


MOEPHOLOGICAL  CHAKACTEKS. 
I.   CRYSTALS  AND  CRYSTAL  SECTIONS. 

Literature. 
G.  WERTHEIM,  Ueber  eine  am  zusaramengesetzten  Mikroskop  angebrachte  Vor- 

richtung  zum  Zweck  der  Messung  in   der  Tiefenrichtung  und  eine  hierauf 

gegriindete  neue  Methode  der  Krystallbestinimung.     S.  AV.  A.  Math.-naturw. 

Classe.  2.  Abth.  Bd.  XLV.  157-170.    1862. 
J.  THOULET,  Precede  pour  mesurer  les  angles  solides  des  cristaux  rnicroscopiques. 

Bull.  Soc.  min.  Fr.  1878.  I.  68. 
E.  BERTRAND,  De  la  mesure  des  angles  diedres  des  cristaux  niicroscopiques.     C.  R. 

LXXXV.  1175.  1877. 
—  De  1'application  du  microscope  a  1'etude  de  la  mineralogie.    Bull.  Soc.  min.  Fr. 

1878.  I.  22. 

THE  extraordinary  importance  of  the  morphological  characters  of 
minerals  for  their  macroscopical  determination  is  greatly  reduced  for 
their  determination  microscopically. 

Complete  crystal  bodies  are  only  seen  under  the  microscope  in  par- 
ticular cases,  as  in  isolated  material  with  very  small  crystals  and  as 
so-called  individualized  interpositions;  in  other  cases  only  the  cross- 
sections  of  crystals  have  to  be  considered,  and  as  the  position  of  these 
sections  bears  no  regular  relation  to  the  crystal  axes,  it  is  readily  seen 
that,  with  the  endless  number  of  possibilities  for  the  cutting  plane, 
neither  the  outline  of  the  cross-section  nor  the  relation  of  its  angles 
are  of  any  absolute  value  in  determining  the  crystal. 

If  the  direction  in  which  the  section  cuts  a  crystal  were  known,  it 
would  not  be  difficult  to  calculate  the  form  of  its  outline  and  its 
angles ;  but  in  most  cases  there  is  no  basis  for  such  a  calculation. 
Where  the  optical  phenomena  establish  the  position  of  the  optical 
constants  in  a  crystal  section,  a  more  or  less  definite  conclusion  can  be 
drawn  as  to  the  position  of  the  section,  by  a  proper  combination  of 
these  with  the  outlines  of  the  cross-sections  and  other  characters,  such 
as  the  course  of  the  cleavage  ;  and  this  conclusion  may  be  so  far  sub- 
stantiated by  angle  measurements  on  the  cross-sections,  that  it  may  be 
considered  as  practically  established. 


MORPHOLOGICAL  CHARACTERS.  5 

Moreover,  a  statistical  method  of  procedure  frequently  furnishes 
valuable  criteria;  for  although  there  may  be  an  endless  number  of 
section  planes,  yet  the  probability  of  the  occurrence  of  all  of  these  is 
not  the  same  for  each,  but  is  essentially  dependent  on  the  relative 
dimensions  of  the  crystals  and  on  the  arrangement  of  them  in  the  rock. 
So  if  those  cross-sections  which  occur  most  frequently  are  properly 
combined,  the  form  of  the  crystal  may  be  determined  without  difficulty. 

If,  for  example,  quadratic  and  six-sided,  colorless  sections  of  a  min- 
eral should  be  found  to  preponderate  over  all  others,  the  interpretation 
would  have  an  exceedingly  wide  scope.  But  if,  on  investigation  in 
polarized  light,  it  should  be  found  that  both  kinds  of  sections  remained 
dark  in  every  position  between  crossed  nicols,  one  would  rightly  con- 
clude that  they  must  belong  to  an  isotropic  or  isometric  mineral  which 
crystallized  in  the  form  of  rhombic  dodecahedrons — mosi  likely  a  min- 
eral of  the  garnet  or  the  haiiyne  groups. 

If,  however,  the  tetragonal  sections  (parallel  OP)  remained  dark 
between  crossed  nicols,  while  the  hexagonal  ones  (parallel  to  the  prin- 
cipal axis  through  a  combination  ooP .  P)  were  generally  light,  they 
would  belong  to  a  mineral  crystallizing  in  the  quadratic  system — 
possibly  to  the  scapolite  group. 

On  the  other  hand,  they  would  belong  to  the  hexagonal  crystal 
system,  if  the  hexagonal  sections  (parallel  OP)  remained  dark  between 
crossed  nicols,  while  the  quadratic  ones  (parallel  to  the  principal  axis 
through  a  combination  ooP  .  OP)  were  generally  light,  and  the  min- 
eral would  probably  be  nepheline  or  apatite. 

If,  finally,  both  quadratic  and  hexagonal  sections  were  for  the  most 
part  light  between  crossed  nicols,  they  might  probably  belong  to  an 
orthoclastic  feldspar,  which  had  been  so  cut  that  the  section  in  one  case 
passed  about  parallel  to  the  orthopinacoid,  and  in  the  other  was  so 
inclined  to  the  base  as  to  pass  through  the  anterior  and  posterior  prism 
faces. 

It  need  scarcely  be  mentioned  that  the  correctness  of  these  con- 
clusions in  many  cases  may  be  placed  beyond  doubt  by  further  optical 
determinations,  by  measurements  of  angles,  by  comparison  of  the 
microstructure  of  the  questionable  cross- sections  with  those  of  known 
material,  and  finally  by  microchemical  tests. 

The  measurements  which  may  be  made  with  the  microscope  relate 
either  to  linear  extension,  to  plane  angles,  or  to  solid  angles.  Linear 
extension  is  measured  by  means  of  an  ocular  micrometer.  This 
consists  of  a  glass  plate  on  which  a  sufficiently  fine  scale  has  been 
>engraved  with  a  diamond.  Generally  a  millimetre  is  divided  into  10 


6  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

parts,  whole  millimetres  being  separated  by  long  marks,  half  ones  by 
medium  lines,  and  tenths  by  short  ones.  With  an  ocular  micrometer 
one  does  not  measures  the  object  itself,  but  its  image.  In  order  to  de- 
termine the  actual  value  of  a  division  of  the  ocular  micrometer  for  a 
particular  system  of  objectives,  a  glass  plate  with  fine  divisions  (object 
micrometer)  is  placed  under  the  objective,  and  the  relation  between 
the  two  scales  is  established.  If  the  ocular  micrometer  is  divided  into 
tenths  of  a  millimetre,  and  the  object  micrometer  into  hundredths  of  a 
millimetre,  and  three  divisions  of  the  former  cover  one  division  of  the 
latter,  then  with  this  system  of  lenses  one  division  of  the  ocular  microm- 
eter will  correspond  to  an  actual  extent  in  the  object  of  0.0033  mm. 

The  measurement  of  a  plane  angle  is  made  by  placing  the  apex  of 
the  angle  to  be  measured  on  the  centre  of  the  cross-wires  in  the  ocular ; 
and  since  the  stage  of  a  petrographical  microscope  is  made  to  rotate 
accurately  about  the  optical  axis  of  the  microscope,  the  sides  of  the 
plane  angle  are  covered  in  turn  by  the  cross-wires,  and  the  amount  of 
rotation  is  read  off  on  the  graduated  circle  of  the  stage. 

The  measurement  of  solid  angles,  which  is  fully  treated  in  the 
German  edition  and  by  the  authors  already  cited,  is  here  omitted. 

II.     NORMAL    AND    ABNORMAL    CRYSTALLIZATION. 

a.  The  External  Form. 

Literature. 
H.  BEHRENS,  Die  Krystalliten.     Mikroskopische  Studien  liber  verzogerte  Krystall- 

bildung.     Kiel.  1874. 
M.  L.  FRANKENHEIM,  Ueber  das  Entstelien  und  das  Wachsthum  der  Krystalle  nach 

mikroskopischen  Beobachtungen.     Pogg.  Ann.  CXI.  1860. 
O.  LEHMANN,  Ueber  physikalische  Isomerie.     Z.  X.  1877.  I.  97-131. 

—  Ueber  das  Wachsthum  der  Krystalle.     Z.  X.  1877.  I.  453-496  (auch  als  Beilage 

zum  Programm  des  Gymnasium  zu  Freiburg  i.  B.  1877). 
F.  LEYDOLT,  Ueber  die  Krystallbildung  im  gewohnlichen  Glase  und  in  den  ver- 

schiedenen  Glasfliissen.     S.  W.  A.  1852.  Math. -nature.  Classe  VIII.  261-277. 
LINK,  Ueber  die  Bildung  der  festen  Korper.     Berlin.  1841. 
H.  VOGELSANG,  Ueber  die  mikroskopische  Structur  der  Schlacken  und  liber  die 

Beziehungen  der    Mikrostructur  zur   Genesis   der   krystallinischen   Gesteine. 

Pogg.  Ann.  1864.  CXXI.  101-125. 
-  Philosophic  der  Geologic.     Bonn.  1867. 

—  Sur  les  crystallites.     Arch.  Neerland.  V.  1870 ;  VI.  1871  ;  VII.  1872. 

• —  Die  Krystalliten.  Nach  dem  Tode  des  Verfassers  herausgegeben  von  F.  ZIRKEL. 
Bonn.  1875. 

IF  an  aqueous,  molten,  or  gaseous  solution  contains  crystallizable 
compounds  under  conditions  (saturation)  which  make  their  separation 
or  secretion  possible,  the  development  of  crystals  will  begin  when 
there  is  sufficient  mobility  of  the  molecules,  and  will  continue  as  long 


SPACE  OF  CRYSTALLIZATION.  7 

as  the  conditions  are  favorable  for  their  separation  and  regular  group- 
ing. Their  number  and  size  will  depend  on  the  number  of  centres  of 
crystallization  and  the  quantity  of  the  material  contributing  to  the 
growing  crystal. 

Every  growing  crystal  will  exert  an  attracting  and  directing  influ- 
ence upon  those  molecules  in  the  solution  which  are  capable  of  enter- 
ing into  the  composition  of  the  crystal,  and  are  within 'the  sphere  of 
its  molecular  forces ;  and  it  will  grow  through  their  accession  and  ar- 
rangement. In  this  way  there  arises  about  every  growing  crystal  a 
mantle  of  solution  which  is  poorer  in  matter  pertaining  to  the  crystal, 
and  to  which  crystallizable  molecules  are  constantly  being  supplied 
through  diffusion  out  of  the  saturated  mother-liquor,  and  from  which 
they  are  being  withdrawn  by  their  incessant  addition  to  the  crystal. 

So  long  as  this  process  continues  normally,  the  growing  crystals 
will  be  bounded  in  every  stage  of  their  growth  by  continuous  plane 
faces.  If  we  transfer  this  process  to  a  molten  solution,  and  imagine 
that  the  condition  of  saturation  ceases  with  respect  to  a  substance  sep- 
arating out,  and  that  at  about  the  same  time  the  movement  of  the 
molecules  is  gradually  hindered  by  the  increasing  viscosity  of  the  solu- 
tion, then  at  some  particular  moment  the  diffusion  of  the  crystallizable 
compound  from  the  mother-liquor  into  the  space  of  crystallization 
(Krystallisationshof )  (PI.  I.  Fig.  1)  would  cease;  yet  in  the  immediate 
vicinity  of  the  crystal,  thaAis,  within  this  space,  so  much  heat  is  liber- 
ated by  the  passage  of  the  accessory  molecules  into  a  solid  state,  that 
crystallizable  molecules  within  this  space  can  attach  themselves  to  the 
crystal  from  the  space  of  crystallization. 

Consequently,  after  the  complete  cessation  of  crystallization  the 
space  of  crystallization  will  be  noticeably  poorer  in  the  crystallizable 
compound  than  the  mother-liquor.  If  there  was  a  tendency  in  this 
compound  to  color  the  mother-liquor,  then  after  the  solidification  of 
the  whole  the  crystal  will  be  surrounded  by  a  space  which  is  lighter 
colored  than  the  mother-liquor.  This  phenomenon  is  often  observed  in 
porphyritic  rocks,  and  PI.  II.  Fig.  6  exhibits  the  same  around  augite  in 
the  obsidian  of  Hammarsfjord.  If  the  centres  of  crystallization  are 
sufficiently  far  apart,  and  the  process  of  crystallization  ends  while  there 
is  mother-liquor  still  present,  then  the  boundaries  of  the  crystals 
formed  will  be  essentially  determined  by  their  proper  laws  of  forma- 
tion (morphology) ;  if  one  or  both  of  the  above  conditions  are  not  ful- 
filled, then  the  perfecting  of  each  single  individual  will  be  hindered 
and  disturbed  by  those  lying  next  to  it,  and  there  will  result  a  more 
or  less  irregular  crystalline  aggregate. 


8 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


M 


If  the  accession  of  matter  through  the  diffusion  currents  into  the 
space  of  crystallization  is  very  abundant  and  accelerated,  then  certain 
parts  of  the  crystal,  particularly  those  to  which  a  greater  portion  of 
this  space  is  tributary,  will  grow  faster  than  other  parts.  O.  Lehmann 
has  called  attention  to  the  fact  that  in  the  growth  of  a  crystal  the 
edges  and  corners  would  have  an  advantage  over 
the  same-sized  part  of  the  faces,  as  is  illustrated 
in  Fig.  1,  in  which  ah,  be,  cd,  de,  ef,  etc.,  repre- 
sent equal-sized  portions  of  the  faces  of  a  growing 
crystal,  and  A,  B,  C,  etc.,  the  part  of  the  space  of 
crystallization  tributary  to  each  portion  of  the 
faces. 

Fig.  ia  If  the  growth  of  the  crystal  ceases  during  the 

exuberant  growth  of  the  points  and  edges,  then  its  outline  will  be  in 
the  form  of  a  ruin  or  of  steps,  or  will  be  indented,  as  Fig.  1#  shows ; 
.but  each  boundary  element  will  be  parallel  to  every  other  equivalent 
boundary  element.  Such  forms  are  quite  frequent  phenomena  among 
the  feldspars,  augites,  hornblendes,  olivines,  etc.,  of  porphyritic 
eruptive  rocks. 

The  growth  of  a  crystal  takes  place  in  essentially  the  same  manner, 
as  soon  as  any  of  the  above-mentioned  normal  conditions  of  growth  are 
in  any  way  disturbed.  If  the  necessary  mobility  of  the  crystallizable 
molecules  in  the  solution  is  wanting  becaus^  of  the  too  rapid  evapora- 
tion of  the  solvent,  or  because  of  its  too  great  viscosity,  its  too  strong 
adhesion  to  the  containing  walls  (object-glass  and  cover-glasses,  for 
instance),  or  from  any  other  circumstances  ;  or  if  the  mobility  ceases 
too  soon,  or  if  the  necessary  saturation  with  the  crystallizable  com  pound 
is  lacking  in  any  or  in  all  parts  of  the  space  of  crystallization, — then 
there  must  occur  disturbed  crystallizations,  and  forms  arise  which  are 
designated  in  general  as  forms  of  growth. 

In  porphyritic  and  glassy  eruptive  rocks,  forms  of  growth  which 
have  been  produced  by  too  great  viscosity  of  the  magma  are  of  frequent 
occurrence.  The  explanation  and  most  of  the  nomenclature  of  these 
extremely  variable  structures  are  derived  from  the  studies  of  H. 
Vogelsang,  and  have  been  elaborated  by  the  work  of  O.  Lehmann. 

If  a  solution  of  sulphur  in  carbon  bisulphide,  which  has  been 
thickened  with  Canada  balsam,  is  spread  on  an  object-glass,  in  a  short 
time  larger  and  smaller  spherules  separate  .out,  which  are  strongly  re- 
fracting and  are  saturated  drops  of  the  sulphur  solution.  By  evapora- 
tion these  lose  their  solvent,  and  finally  become  solid.  Vogelsang  saw 
in  these  amorphous,  round,  drop-like  forms  the  elementary  bodies  of 


MICROLITIC  STRUCTURES.  9 

crystals,  and  called  them  Globulites  (PI.  I.  Figs.  1  to  4).  If  the  solution 
dries  up  about  the  time  the  globnlites  are  formed,  they  suffer  no 
further  change.  But  if  the  solution  preserves  sufficient  mobility  for 
some  time,  currents  set  in,  by  which  the  globulites  change  their  place, 
and  are  sometimes  aggregated  in  quite  irregular  heaps,  Cumulites  (PL 
I.  Fig.  5),  sometimes  in  more  or  less  regular  structures.  Frequently 
they  arrange  themselves  in  rows  like  strings  of  pearls  (PL  I.  Fig.  3), 
which  Vogelsang  called  Margarites. 

Moreover,  globulites  increase  in  volume  by  coalescing  with  one  an- 
other (PL  I.  Fig.  2).  So  long  as  the  resistance  of  the  solvent  is  not  too 
great  the  enlarged  globulite  retains  the  spherical  form  ;  otherwise  there 
arise  cylindrical,  disk-like,  or  sharply  conical  and  crooked  forms,  which 
are  classed  together  as  Longulites  (PL  I.  Fig.  3).  Globulites  and 
longulites,  as  well  as  their  manifold  aggregations  with  one  another, 
do  not  possess  the  characteristics  of  crystals.  Vogelsang  named 
them  collectively  Crystallites,  and  found  them  singly  refracting  so- 
long  as  their  elements  preserved  the  globulitic  form,  or  their  more 
complex  forms  did  not  exceed  the  stage  of  globulitic  aggregation. 

With  sufficient  mobility  of  the  solution  the  supersaturated  drop  or 
globulite  does  not  solidify  as  such,  but  takes  the  form  of  the  ortho- 
rhombic  sulphur  pyramid  at  the  instant  of  solidification.  This  is 
especially  noticeable  when  the  globulites  have  been  driven  by  the  cur- 
rents on  to  normally  developed  sulphur  crystals  or  forms  of  growth, 
with  which  they  have  grown  into  skeleton  crystals  with  several  axes. 
Upon  the  loss  of  the  globulitic  form  and  the  accession  of  a  crystal- 
lographic  boundary  double  refraction  regularly  appears. 

The  researches  of  H.  Vogelsang  and  his  successors  explain  those 
appearances  in  rocks  which  are  so  closely  related  to  the  artificial  pro- 
ductions. These  products  of  incomplete  crystallization  naturally  occur 
in  the  more  or  less  basic  porphyritic  eruptive  rocks  which  have  not 
reached  a  holocrystalline  development.  Indeed  in  many  of  these  rocks 
the  residuum  of  crystallization,  called  the  base,  is  entirely  made  up  of 
these  kind  of  crystal  structures.  The  globulites  (PL  I.  Fig.  4),  which 
in  rocks  poor  in  silica  are  generally  strongly  colored  or  opaque,  and  in 
the  siliceous  rocks  are  usually  clear  and  transparent,  occur  uniformly 
disseminated,  or  strung  together  into  margarites  (PL  II.  Fig.  1).  In 
place  of  the  round  or  disk-shaped  globulites,  or  beside  them,  are  lon- 
gulites ;  and  the  closer  study  of  many  obsidians  and  vitrophyres  reveals 
an  endless  variety  of  all  imaginable  intermediate  forms  between  the 
loosely  strung  margarites  and  crystal  needles.  The  crowding  together 
of  globulites,  and  irregularly  massed  or  more  or  less  regularly  arranged 


10  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

groups,  are  confined  principally  to  the  most  acid  siliceous  rocks  (quartz- 
porphyries  and  rhyolites).  Cumulites  in  which  there  is  a  radial 
arrangement  of  the  single  globulites  are  called  Globospherites  (PI.  I. 
Fig.  6).  Since  in  such  globosphe  rites  there  is  a  constant  diminution 
in  density  from  the  centre  toward  the  periphery,  interference  phenom- 
ena appear  in  certain  instances  in  polarized  light  which  are  analogous 
to  those  in  spherulites  (spharokrystalle). 

Those  crystalline  structures,  or  forms  of  growth,  which  have  de- 
veloped beyond  the  stage  of  globnlitic  aggregates  but  have  not  attained 
complete  perfection  of  form,  are  exceedingly  manifold.  With  all 
their  variation  in  appearance,  they  agree  in  that  they  are  not  composed 
of  elementary  bodies,  and  in  that  they  often  possess  the  physical 
characters  of  crystals  in  a  recognizable  manner,  or  permit  them  to  be 
conjectured  from  their  form.  Their  most  important  forms  may  be 
characterized  as  microlitic  structures. 

Trichites  (6pi£  =  hair),  according  to  Zirkel,  are  hair-like  crystals 
whose  length  greatly  exceeds  their  breadth ;  they  are  often  more  or 
less  twisted,  and  even  bent  in  loops.  They  have  a  great  tendency  to 
arrange  themselves  in  many-armed  groups  about  a  central  grain  (PI.  II. 
Fig.  2).  They  usually  appear  opaque,  because  of  their  small  diameter 
and  the  consequent  total  reflection  of  transmitted  light. 

/Spherulites  (spharokrystalle)  form  another  group  of  incipient  forms 
of  growth  closely  related  to  trichites.  They  include  a  great  part  of  the 
spherulites  which  occur  in  different  porphyritic  rocks.  They  are 
homogeneous  spherical  crystal  structures,  which  are  radially  fibrous,  in 
some  cases  with  a  rough  surface,  in  others  with  a  more  or  less  smooth 
one  (PL  II.  Fig.  4).  The  needles  composing  a  spherulite  are  not 
always  simple  crystal  needles,  but  are  sometimes  many-branched  forms 
arising  from  .the  repeated  splitting  of  a  simple  needle  into  two  or  more 
slightly  diverging  arms,  which  are  in  turn  split  up.  PI.  III.  Fig.  4 
shows  a  variety  of  spherulites  of  feldspar  in  a  trachytic  rock  from 
the  Caucasus. 

While  the  trichitic  structure  appears  in  general  to  be  confined  to 
the  more  basic  rock  constituents  rich  in  iron,  and  therefore  to  relatively 
older  periods  in  the  development  of  a  rock,  the  spherulitic  structure 
belongs  to  the  more  acid,  feldspathic,  or  feldspar-like  constituents, 
poor  in  iron,  and  to  comparatively  late  periods  of  the  rock  formation. 

Skeleton  crystals,  strictly  speaking,  are  those  crystallizations  which 
have  not  produced  entire  and  complete  individuals,  but  have  led  to 
crystallographically  parallel  or  symmetrical  aggregates  of  small  indi- 


MECHANICAL  DEFORMATION.  11 

viduals;  the  latter  may  be  arranged  throughout  their  whole  extent 
as  a  single  individual  or  as  a  twinned  one.  PI.  III.  Fig.  2  shows 
skeleton  crystals  of  magnetite,  and  PI.  III.  Fig.  3  those  of  augite.  PI. 
III.  Fig.  1  shows  an  intermediate  form  between  a  crystal  and  a  skeleton 
crystal  of  olivine. 

The  name  microUte  (piKpoS  =  small ;  Azflos"  =  stone)  may  be  ap- 
plied to  more  or  less  completely  defined  crystals,  without  reference  to 
their  habit  or  to  their  optical  behavior,  and  which  are  only  recognizable 
microscopically,  and  cannot  be  specifically  determined.  If  their  nature 
is  determinable,  they  are  called  by  the  specific  name,  together  with  an 
expression  indicating  their  habit ;  as,  for  example,  lath-shaped  feldspar, 
angite  prisms,  mica  plates,  perofskite  octahedrons,  etc.  Microlites  are 
true  crystals,  as  is  proven  by  their  form. 

Besides  the  deformation  of  crystals  which  has  been  produced  by 
conditions  attending  their  growth,  the  microscope  reveals  a  number  of 
others  which  have  affected  completed  individuals  and  are  due  to  me- 
chanical and  chemical  processes. 

To  the  mechanical  deformation  of  crystals  belong  the  cracking  and 
breaking  apart  of  the  older  secretions,  porphyritic  crystals,  which 
occurs  so  frequently  in  porphyritic  rocks.  It  is  recognized  by  the 
irregular  broken  outline  of  the  mineral  section  across  the  surface  of 
fracture.  Elastic  minerals  like  mica  exhibit  bending  instead  of  breaking. 
PI.  III.  Fig.  5  shows  a  broken  feldspar  crystal ;  PL  III.  Fig.  6  a  bent 
mica  plate. 

Another  group  of  deformations  of  rock  constituents  due  to  me- 
chanical processes  is  met  with,  especially  in  greatly  faulted  and  up- 
lifted mountain  masses,  and  is  evidently  occasioned  by  the  dynamical 
processes  of  mountain-building.  From  the  fact  that  these  deformations 
take  place  in  solid  rock  under  pressures  exerted  on  all  sides,  they  do 
not  generally  appear  as  great  alterations  of  the  outward  form,  but  more 
as  internal  displacement  of  parfcs  of  a  crystal  with  respect  to  one  an- 
other. These  deformations,  therefore,  are  often  first  recognized  in 
polarized  light  by  the  greater  or  less  variation  of  the  optical  orientation 
of  the  parts  of  the  crystal. 

Examples  of  this  kind  of  deformation  are  found  in  the  bending  of 
the  twin  lamellae  of  triclinic  feldspar  (PL  IY.  Fig.  6) ;  in  the  varying 
position  of  the  axes  of  elasticity  in  particular  parts  of  feldspar  and 
quartz  crystals,  which  shows  itself  by  the  shadowy  and  rapidly  shifting 
extinction  over  the  section  during  its  rotation  between  crossed  nicols. 
Through  greater  pressure  the  crystal  may  be  more  or  less  broken  up 


12  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

(PI.  IV.  Figs.  1,  2),  or  the  crystal  outline  disappear  altogether  (PI.  FV. 
Figs.  8,  4). 

It  has  been  very  frequently  observed  that  a  rock  constituent  which 
in  its  normal  condition  is  optically  uniaxial,  becomes  biaxial  after, 
undergoing  great  pressure.  The  natural  occurrence  of  pressure  figures 
in  mica  plates  may  also  be  referred  to  this  cause  (PL  IY.  Fig.  5). 

Chemical  deformations  appear  in  many  ways  among  the  older 
secretions  of  porphyritic  eruptive  rocks.  It  lies  in  the  conception  of 
a  crystal  and  in  the  conditions  under  which  porphyritic  secretions  are 
formed,  that  they  should  possess  regular  cry  stall  ographic  boundaries. 
When  therefore  the  normal  outward  boundary  is  wanting,  it  must 
have  been  lost  through  secondary  action.  Since  the  production  of 
porphyritic  secretions  belongs  to  an  early  stage  in  the  history  of  a  rock, 
and  follows  the  laws  which  obtain  for  crystallization  from  a  mixed 
solution,  it  is  possible  to  imagine  that  through  changes  in  the  chemical 
composition  or  physical  condition  of  the  mother-liquor  (rock  magma) 
the  older  secretions  are  no  longer  able  to  exist,  but  must  dissolve  again 
to  make  room  for  other  crystallizations.  The  older  secretions  are 
therefore  melted  again,  and  if  the  process  of  resorption  is  interrupted 
by  further  changes  or  by  the  solidification  of  the  magma  before  their 
complete  fusion,  there  result  rounded  grains  in  place  of  the  sharp-edged 
crystals.  Often  this  corrosion  in  its  earliest  stages  appears  to  have 
been  one-sided,  as  is  shown  for  quartz  (PL  Y.  Fig.  1),  and  for  nosean 
(PI.  V.  Fig.  2). 

If  the  crystal  substance  which  is  corroded  and  dissolved  by  the 
magma  is  converted  immediately  into  new  crystalline  forms,  as  is  so 
often  the  case  with  the  micas  and  hornblendes  of  eruptive  rocks,  there 
arise  no  properly  corroded  crystals,  but  pseudomorphs  after  the  dis- 
solving crystals  which  grow  from  the  border  inward,  and  which  will 
be  described  in  another  place. 

J.  The  Internal  Structure,  or  Homogeneity. 
Literature. 

DAVID  BREWSTER,  On  the  existence  of  two  new  fluids,  etc.     Transactions  of  the 

Royal  Society  of  Edinb.  T.  X.  1,  Auszug  daraus  Edinb.  Phil.  Journ.  vol.  IX. 

94  u.  268.  —  Sehr  vollstandiger  Auszug,  z.  Th.  Uebersetzung  in  Pogg.  Ann. 

VII.  1826.  469. 
HUMPHRY  DAVY,  On  the  state  of  water  and  aeriform  matter  in  cavities,  found  in 

certain  crystals.     Philos.  Transactions  1822,  in  franzosischer  Uebersetzung  in 

Annales  de  chimie  et  de  phys.  T.  XXI.  1822.  132. 
TH.  ERHARD  und  ALFR.  STELZNER,  Ein  Beitrag  zur  Kenntniss  der  Fliissigkeits- 

einschlusse  irn  Topas.     T.  M.  P.  M.  I.  1878.   450-458. 


ZONAL  STRUCTURE.  13 

C.  W.  GUMPEL,  Ueber  die  mit  einer  Fliissigkeit  erfullten  Chalcedonmandeln  (En- 
hydros)  von  Uruguay.  S.  M.  A.  1880.  II.  Math.-phys.  Classe.  241-254. 

—  Nachtrage  zu  den  Mittheilungen  tlber  die  Wassersteine  (Enhydros)  von  Uruguay 
uud  ilber  einige  siid-  und  mittelamerikanische  sog.  Andesite.  ibidem  188JL  I. 
321-268. 

W.  N.  HARTLEY,  On  the  presence  of  liquid  carbon  dioxide  in  mineral  cavities. 
Journal  of  the  Chemical  Society.  London.  1876.  I.  137-143. 

—  On  variations  in  the  critical  point  of  carbon  dioxide  in  minerals  and  deductions 

from  these  and  other  parts,   ibidem  1876.  II.  237-250. 

—  Observations  on  fluid  cavities,    ibidem  1877.  I.  241-249. 

—  On  attraction  and  repulsion  of  bubbles  by  heat.     Proceed.  Roy.  Soc.  XXVI.  137. 

1878. 

—  On  the  constant  vibration  of  minute  bubbles,  ibidem  XXVI.  150.  1878. 

G.  W.  HAWES,  On  liquid  carbon  dioxide  in  smoky  quartz.     Amer.  Journ.  1881. 

XXI.  203-209. 
AL.  A.  JULIEN,  On  the  examination  of  carbon  dioxide  in  the  fluid  cavities  of  Topaz. 

Journ.  of  the  Amer.  Chem.  Soc.  III. 
W.  PRINZ,  Les  enclaves  du  saphir,  du  rubis  et  du  spinelle.     Ann.  de  la  Soc.  belg. 

de  microscopic.     1882. 
H.  CL.  SORBY,  On  the  microscopical  structure  of  crystals  etc.     Quart.  Journ.  of  the 

Geol.  Soc  London  1858.  Nov.  vol.  XIV.  453-500  und  andere  Arbeiten  dessel- 

ben  Verfassers,  cf.  Literaturnaelrweis. 
H.  VOGELSANG  und  GEISSLER,  Ueber  die  Natur  der  Flussigkeitseinschliisse  in  ge- 

wissen  Mineralien.     Pogg.  Ann.  vol.  CXXXVII.   1869.  56  und  Nachtrag  zu 

dieser  Abhandlung  von  VOGELSANG,    ibid.  257. 
ARTH.  W.  WRIGHT,  On  the  gaseous  substances  contained  in  the  smoky  quartz  of 

Branchville,  Conn.     Amer.  Journ.  1881.  XXI.  209-216. 

Theoretically,  the  substance  of  a  crystal  should  be  of  unbroken 
continuity  and  perfectly  homogeneous  ;  but  these  qualities  seldom  ex- 
ist together  in  nature. 

Zonal  Structure. — From  the  fact  that  the  growth  of  a  crystal  from 
a  solution  is  not  always  a  single  continuous  act,  but  is  at  times  inter- 
rupted by  longer  or  shorter  intervals  of  inaction,  there  arises  a  shelly 
structure,  which  in  cross-section  produces  the  appearance  called  zonal 
structure. 

This  is  particularly  well  shown  in  many  zircons,  frequently  in  the 
.feldspars  of  trachytes  and  andesites,  and  in  the  nepheline  and  lencite 
of  the  more  basic  lavas  (PL  Y.  Fig.  3). 

Where  the  shelly  individuals  are  minerals  which  may  be  regarded 
as  isomorphous  mixtures  of  several  molecular  groups  (garnet,  tourma- 
line, pyroxene,  am  phi  bole,  mica,  etc.),  then  the  successive  shells  often 
differ  in  chemical  composition.  If  the  isomorphous,  laminated  com- 
pounds are  colored,  the  variation  in  composition  is  frequently  recog- 
nized by  the  different  colors  of  the  separate  zones  (PI.  Y.  Figs.  4  and  5). 
The  form  of  the  different  shells  naturally  depends  on  the  manner  of 
growth  in  the  crystal. 


14  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

The  lines  of  zonal  structure  are  usually  parallel  to  the  outlines  of 
the  crystal,  or  to  certain  of  its  outlines.  Consequently,  if  the  outer 
form  of  a  crystal  has  been  destroyed  by  resolution,  its  character  may  be 
reasonably  inferred  from  the  zonal  structure.  Yet  cases  occur  in 
which  this  parallelism  is  not  present,  and  the  zonal  structure  indicates 
another  crystal  form  from  that  shown  by  the  outline  of  the  individual. 

Since  the  optical  behavior  of  a  substance,  independent  of  pressure 
and  temperature,  is  a  function  of  its  molecular  composition,  it  follows 
that  the  value  and  position  of  the  axes  of  elasticity  and  of  the  optical 
axes,  as  well  as  the  pleochroic  relations,  may  differ  in  the  various  shells 
of  a  crystal  with  isomorphous  lamination. 

Inclusions. — The  term  discontinuity  of  crystal  substance  may  be 
applied  to  a  group  of  phenomena  which  arise  from  the  fact  that  the 
space  occupied  by  a  crystal  is  not  entirely  filled  by  the  crystal  substance, 
but  in  part  by  bodies  foreign  to  it.  All  these  so-called  foreign  bodies 
are  classed  as  inclusions  or  interpositions,  and  may  be  divided  into 
unindividualized  inclusions  and  individualized  inclusions,  according 
as  the  inclusion  consists  of  amorphous  substances  in  any  state  of  aggre- 
gation whatever,  or  of  crystallized  bodies.  The  first  are  produced  by 
the  growing  crystal  taking  up  particles  of  the  magma  or  gases  and  fluids 
contained  in  it ;  the  second  arise  through  the  inclusion  by  the  growing 
crystal  of  pre-existing  crystallizations,  or  of  those  which  were  being 
secreted  at  the  same  time  from  the  magma.  Experience  with  the  artifi- 
cial production  of  crystals  has  shown  that  interpositions  are  taken  up 
more  abundantly  by  growing  crystals  as  their  growth  is  more  rapid. 

According  to  the  character  of  unindividualized  inclusions  they  are 
divided  into  gas  inclusions,  fluid  inclusions,  and  glass  inclusions. 

Gas  Inclusions. — Gas  inclusions  are  also  called  gas  and  vapor  cav- 
ities :  they  are  recognized  chiefly  by  their  outward  appearance.  Because 
of  the  great  differences  in  the  indices  of  refraction  of  solid  and  gaseous 
bodies,  these  inclusions  must  appear  glistening  by  incident  light,  but  in 
transmitted  light  like  small  spots  with  large  dark  borders  (PL  VI. 
Fig.  1).  This  phenomenon,  which  results  from  the  total  reflection  of 
the  rays  of  incident  light,  may  be  observed  on  any  gas  bubbles,  as  those 
which  rise  in  soda-water  or  champagne,  or  occur  too  frequently  in 
the  Canada  balsam  of  thin  sections. 

The  form  of  gas  inclusions  varies  greatly,  but  round  and  elliptical 
shapes  predominate,  along  with  which  occur  irregularly  jagged,  bay- 
shaped,  branching,  and  other  forms.  Less  frequently  they  appear  as 
negative  crystal  cavities,  that  is,  with  a  polygonal  boundary  correspond- 
ing to  the  crystal  form  of  their  host  ( Wirth).  Gas  cavities  seldom 


GAS  INCLUSIONS.  15 

occur  isolated,  but  are  generally  grouped  in  rows  and  planes  through 
the  substance  of  a  mineral,  and  with  low  magnifying  power  appear  as  a 
local  clouding  of  the  mineral. 

The  secretion  of  crystals  can  take  place  from  aqueous  solutions,  and 
from  those  which  are  fluid  when  melted,  or  by  sublimation,  and  in  all 
three  ways  they  may  acquire  gas  cavities.  Their  formation  in  crystals 
resulting  from  sublimation  needs  no  special  explanation.  It  is  well 
known  that  water  at  different  temperatures  absorbs  different  amounts 
of  various  gases ;  from  this  the  enclosure  of  primary  gas  inclusions  fol- 
lows naturally. 

Secondary  gas  cavities  can  occur  in  certain  hydrogenous  crystals,  if 
original  fluid  inclusions  evaporate,  as  not  infrequently  happens  in  the 
case  of  minerals  with  very  perfect  cleavage. 

The  great  capacity  of  melted  fluids  to  dissolve  gases  is  an  established 
fact.  As  soon,  however,  as  cooling  sets  in  with  the  consequent  solidi- 
fication the  absorbed  gas  must  be  liberated  ;  hence  the  "  sprouting"  of 
silver,  the  porous  structure  of  lavas,  etc.  This  explains  the  presence  of 
gas  cavities  in  glassy,  solidified  fluids,  like  obsidian,  and  in  pyrogenous 
minerals,  like  nepheline  and  others.  There  is  little  definite  knowledge 
concerning  the  chemical  nature  of  the  gases  filling  such  cavities ;  or 
whether  they  are  always  filled  with  gas,  and  are  not  sometimes  in  the 
case  of  glassy  bodies  simply  contraction  phenomena.  Whether  the 
cavities  are  filled  with  gas,  and  with  what  kind,  depends  also  upon  the 
permeability  or  impermeability  of  the  walls  of  the  cavities.  The  ame- 
thyst of  Schemnitz  has  been  found  to  be  impermeable  to  gases,  while 
calcite  is  always  found  to  be  permeable. 

If  the  enclosed  gases  possess  a  certain  tension,  or  possessed  it  at 
the  time  of  their  inclusion,  while  the  enclosing  body  had  a  certain 
plasticity  of  substance,  as  with  glasses  and  other  amorphous  bodies, 
then  the  pressure  exerted  by  it  would  induce  a  molecular  strain  in  the 
solid  substance,  which  might  lead  to  the  phenomena  of  double  refrac- 
tion, which  do  not  otherwise  occur  in  amorphous  bodies,  but  which 
may  be  produced  in  them  artificially  by  the  application  of  external 
pressure.  A  similar  disturbance  of  the  normal  optical  properties  of  a 
crystallized  matrix  also  may  result  from  the  tension  of  enclosed  gases. 

Fluid  Inclusions. — Fluid  inclusions,  like  gas  inclusions,  occur 
mostly  in  groups,  and  like  them  are  usually  arranged  in  lines  and 
along  planes,  which  in  some  cases  pass  irregularly  through  the  crys- 
tal, in  ohers  are  arranged  more  or  less  closely  in  accord  with  the  crys- 
tallographic  constants. 

The  shape  of  fluid  inclusions  is  extremely  variable.     Besides  the 


16  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

round  and  elliptical  forms,  which  are  most  frequent,  are  cylindrical, 
club-shaped,  pear-shaped,  quite  irregular,  and  often  branched  forms. 
l^Tot  infrequently  they  have  a  plane  polyhedral  boundary,  which  is 
determined  by  the  crystal  form  of  their  host.  Thus  the  fluid  inclu- 
sions in  rock  salt  are  cubical,  in  calcite  often  rhombohedral,  in  quartz 
dihexahedral,  and  so  on  (PL  VI.  Figs.  2  and  3).  Their  dimensions 
vary  greatly,  so  that  in  the  same  crystal,  besides  fluid  inclusions,  which 
may  be  recognized  as  such  with  the  naked  eye,  are  those  which  when 
highly  magnified  appear  only  as  a  clouding  of  the  mineral  substance. 

Since  the  index  of  refraction  of  fluids  differs  less  from  that  of  solid 
bodies  than  that  of  gases  does,  the  dark  border  about  fluid  inclusions 
produced  by  the  total  reflection  of  the  transmitted  light  will  be  general- 
ly narrower  than  that  about  gas  inclusions.  But  sometimes  the  indices 
of  refraction  of  the  fluid  and  of  the  mineral  containing  it  are  very 
different ;  moreover,  the  breadth  of  the  dark  border  depends  not  only 
on  the  relative  indices  of  refraction  of  the  two  substances,  but  on  the 
shape  of  the  fluid  inclusion,  whose  bounding  plane,  if  greatly  inclined 
to  the  line  of  vision,  may  produce  a  border  as  broad  as  that  of  a  gas 
inclusion  ;  therefore  the  distinction  based  on  this  character  is  not  abso- 
lutely certain. 

The  fluid  may  either  completely  or  partially  fill  the  cavity  in  which 
it  occurs,  as  shown  in  Figs.  2  and  3,  PI.  VI.  In  the  latter  case  the 
appearance  differs  with  the  ratio  between  the  amount  of  fluid  and  the 
size  of  the  cavity.  If  there  is  much  fluid  present,  so  that  the  cavity  is 
nearly  filled,  then  the  remainder  will  be  occupied  by  the  vapor  of  the 
fluid,  or  by  another  gas  in  the  form  of  a  round  bubble.  The  border  of 
this  bubble  within  the  fluid  is  broad  ;  that  of  the  fluid  within  the  solid, 
narrow.  If,  however,  the  volume  of  the  fluid  is  quite  small  compared 
with  that  of  the  cavity  containing  it,  and  if  the  fluid  does  not  wet  the 
substance  of  the  crystal  (PL  VI.  Fig.  6),  then  the  fluid  forms  a  drop 
surrounded  by  an  envelope  of  its  vapor  or  of  another  gas.  In  this  case 
the  fluid  apparently  forms  a  bubble  with  dark  border  next  to  the 
vapor  envelope,  which  in  turn  is  bounded  by  a  still  broader  margin. 
The  presence  of  a  bubble  naturally  prevents  the  confusion  of  fluid 
and  gas  inclusions. 

The  bubbles  in  fluid  inclusions  may  arise  from  the  contraction  of 
the  fluid  after  its  enclosure  in  the  crystal,  or  from  the  condensation  of 
vapor  after  its  inclusion,  or  be  due  to  the  fact  that  both  vapor  and  fluid 
were  imprisoned  at  the  same  time.  In  the  first  instance  there  should 
be  a  constant  ratio  between  the  volume  of  the  bubble  and  the  fluid  in 
all  the  inclusions  of  one  individual,  which  is  seldom  observed  in  nature. 


FLUID  INCLUSIONS. 

The  fluids  included  in  crystals  are  almost  always  colorless  ;  occasion- 
ally they  have  a  yellowish  color>  and  rarely  an  orange  color.  The  bub- 
bles occurring  in  fluid  inclusions  often  show  a  spontaneous  movement, 
in  some  cases  swinging  slowly  back  and  forth,  in  others  hurrying  about 
in  a  lively  dance.  The  mobility  appears  to  be  greater  the  smaller  the 
bubble;  large  bubbles  generally  remaining  stationary.  A  motion 
may  be  produced  artificially  in  many  cases  by  heating  one  end  of  the 
inclusion. 

The  mobility  of  the  bubbles  is  a  sufficient  proof  of  the  fluid  con- 
dition of  the  inclusions  in  which  they  occur,  and  constitutes  an  impor- 
tant distinction  between  these  and  glass  inclusions.  It  is  not  to  be 
assumed  that  all  fluid  inclusions, in  minerals  are  primary,  for  it  is  easy 
to  imagine  that  original  gas  inclusions,  or  secondary  cavities  produced 
by  chemical  action,  may  be  filled  with  fluid  through  capillary  crevices. 

Chemical  and  physical  investigation  of  the  contents  of  fluid  and 
gas  inclusions  has  shown  that  they  vary  greatly,  both  as  to  the  nature 
of  the  material  and  the  tension  under  which  it  exists.  The  fluid  is 
usually  water,  carrying  more  or  less  of  other  substances  in  solution ; 
in  some  cases  it  is  petroleum.  The  gas  has  sometimes  the  composition 
of  ordinary  air;  is  often  carbon  dioxide,  nitrogen,  or  a  mixture  of 
gases.  Instances  are  frequent,  especially  in  the  quartz  of  granites  and 
crystalline  schists,  in  rock  crystals,  topaz,  beryl,  etc.,  where  the  fluid 
inclusions  contain  double  bubbles,  one  within  the  other.  These  have 
been  shown  to  consist  of  liquid  and  gaseous  carbon  dioxide  in  water, 
the  water  wetting  the  walls  of  the  cavity,  and  the  liquid  and  gaseous 
carbon  dioxide  occupying  the  central  part  of  the  space ;  the  liquid 
carbon  dioxide  envelops  the  gaseous  when  the  amount  of  the  former 
is  relatively  great,  and  both  take  the  spheroidal  form.  On  the  other 
hand,  the  gaseous  carbon  dioxide  separates  the  liquid  carbon  dioxide 
from  the  water  when  the  relative  proportions  are  reversed.  The  posi- 
tion of  the  broad  and  narrow  borders  produced  by  total  reflection 
generally  distinguishes  these  two  cases  from  one  another. 

The  presence  of  crystalline  secretions  of  various  kinds  in  the  fluid 
inclusions  of  very  different  minerals  has  been  confirmed  by  many 
observers.  The  strikingly  widespread  occurrence  of  cube-like  crystals 
in  the  fluid  inclusions  of  quartz  of  the  greatest  variety  of  crystalline 
rocks  and  in  many  other  minerals  is  specially  to  be  noted  (PL  "VI. 
Fig.  5).  These  are  probably  sodium  chloride  in  some  cases,  but  they 
cannot  always  be  referred  to  this  mineral. 

The  conditions  under  which  crystalline  bodies  separate  out  of  fluid 
inclusions  are  quite  analogous  to  those  under  which  a  glass  inclusion  is 


18  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

converted  into  a  devitrified  inclusion.  In  the  case  of  fluid  inclusions 
they  may  be  briefly  summarized  as  physical  changes  in  the  fluid  which 
prevent  its  retaining  the  dissolved  salts  longer  in  solution.  In  exactly 
the  same  manner  crystalline  separation  may  take  place  out  of  gas  cavi- 
ties when  the  enclosed  sublimation  products  cool. 

Glass  Inclusions. — Solidified  portions  of  the  once  molten  magma  are 
often  found  enclosed  in  minerals  which  crystallized  out  of  melted  solu- 
tions. These  are  called  glass  inclusions  (glass  cavities :  Sorby)  when  the 
solid  is  amorphous,  and  slag  inclusions  (stone  cavities  ;  Sorby)  when  it 
has  a  more  or  less  crystalline  development,  whether  this  accompanied 
the  consolidation  of  the  inclusion  or  was  subsequent  to  it.  The  shape 
of  these  glass  and  slag  inclusions  is  just  as  irregular  and  manifold  as 
that  of  gas  and  fluid  inclusions,  and  they  often  possess  the  form  of 
their  host  (PL  VII.  Figs.  1  and  2),  The  color  of  glass  inclusions  varies 
with  their  chemical  composition,  and  especially  with  the  iron  percentage 
of  the  rock  glass.  They  are  usually  colorless  when  they  occur  in  the 
minerals  of  the  acid  eruptive  rocks,  but  are  very  often  colored  yellow, 
red,  or  brown  in  those  of  basic  rocks.  Very  frequently  these  glass  inclu- 
sions contain  one  or  more  darkly  margined  bubbles,  which  are  not- 
moved  by  changes  of  temperature  ;  and  often  the  glass  particle  is  fairly 
riddled  by  a  great  number  of  bubbles.  The  immobility  of  the  bub- 
bles and  the  presence  of  several  in  one  glass  inclusion  are  the  best  dis- 
tinctions between  these  and  fluid  inclusions  (PL  VII.  Fig.  3).  The 
occurrence  of  bubbles  in  glass  inclusions  arises  from  the  presence  of 
gases  in  the  molten  magma,  which  were  enclosed  along  with  the  glass. 

Individualized  Inclusions. — The  occurrence  of  individualized  in- 
clusions, inclusions  of  one  mineral  in  another,  was  a  well-known  fact 
in  the  case  of  transparent  minerals  before  the  introduction  of  the 
microscope.  Microscopical  investigation  has  only  demonstrated  the 
very  wide  distribution  of  this  kind  of  interpositions,  and  placed  in  a 
clear  light  their  significance  for  the  results  of  chemical  analyses. 
Many  optical  phenomena  also  have  been  explained  by  their  presence, 
as  the  schillerization  of  crystals,  asterism,  etc.  Only  those  foreign  crys- 
tals which  are  older  than  the  enclosing  mineral  or  are  contemporane- 
ous with  its  growth  are  called  interpositions.  Infiltrations  in  cracks 
and  products  of  the  decomposition  and  alteration  of  a  mineral  are  not 
considered  inclusions. 

In  many  cases  there  is  no  particular  relation  between  the  arrange- 
ment of  crystalline  interpositions  and  their  crystal  host  (PL  VII.  Fig. 
4).  Yet  we  know  from  the  macroscopic  parallel  growth  of  many 
minerals  (rutile  with  hematite,  hematite  with  mica,  etc.),  as  well  as 


INDIVIDUALIZED  INCLUSIONS.  19 

through  the  investigations  of  Frakenheim  on  crystallization,  that  a  crys- 
tal can  exert  a  directing  influence  on  crystals  of  a  different  kind  which 
grow  upon  it.  There  is  also  frequently  recognized  among  microscopic 
-crystalline  interpositions  a  definite  arrangement  of  these  with  respect 
to  one  another  and  their  host  (PL  VII.  Fig.  5). 

Another  kind  of  orderly  arrangement  of  inclusions  is  determined, 
not  by  a  crystallographically  directing  force,  but  by  mechanical  con- 
ditions, namely,  the  rate  of  growth.  It  is  their  accumulation  in  certain 
parts  of  the  host  while  other  parts  of  it  are  relatively  or  entirely  free 
from  them.  This  regularity  applies  to  all  varieties  of  inclusions. 
Three  kinds  of  orderly  arrangement  are  recognized :  central  (PI.  VII. 
Fig.  6),  peripheral  (PI.  VIII.  Fig.  1),  and  zonal  (PL  VIII.  Fig.  2). 
In  the  central  arrangement  the  inner  portion  of  the  crystal  is  full  of 
inclusions,  the  outer  more  or  less  free  from  them.  In  the  peripheral 
the  case  is  the  reverse.  In  the  zonal  arrangement  the  inclusions  lie  on 
the  surface  of  concentric  shells  of  the  crystal. 

The  amount  of  individualized  inclusions  in  a  crystal  is  often  so 
great,  that  one  may  speak  of  a  mutual  penetration  of  two  or  more 
materially  and  morphologically  different  substances.  Such  a  mutual 
penetration  of  quartz  and  acid  feldspars  is  especially  common :  it  pre- 
sents a  peculiar  appearance,  characteristic  of  certain  members  of  the 
quartz-porphyry  group,  and  is  the  so-called  micropegmatite  or  grano- 
phyre  structure  (PL  VIII.  Fig.  3).  The  same  intergrowth  is  frequent- 
ly observed  between  different  members  of  the  feldspar  group  (micro- 
cline,  albite,  orthoclase)  in  the  older  massive  rocks,  where  it  is  usually 
controlled  by  rigid  mutual  crystallographic  relations.  The  basic  mas- 
sive rocks  also  exhibit  similar  phenomena,  as,  for  example,  when  the 
larger  porphyritic  augites  are  so  filled  with  apatite,  magnetite,  mica, 
nepheline,  hatiyne,  etc.,  that  the  augite  substance  only  forms  a  cement, 
as  it  were,  for  the  different  minerals  (PL  VIII.  Fig.  6).  This  structure 
has  been  called  poicolitic. 


c.  Twins. 

The  twinning  of  minerals  is  recognized  microscopically  either  by 
the  occurrence  of  reentrant  angles  in  the  outline  of  the  section  or  by 
optical  phenomena.  The  occurrence  of  reentrant  angles  only  character- 
izes certain  varieties  of  twinning,  and  then  is  only  observed  when  the 
outlines  of  the  crystals  are  regular.  The  optical  phenomena  in  polarized 
light  prove  the  presence  of  twinning  in  all  cases,  except  in  minerals  of 
the  isometric  system  and  in  certain  twins  with  parallel  axes.  The  dis- 


20  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

cussion  of  the  optical  phenomena  in  twinned  crystals  will  be  found  in 
a  later  part  of  the  book. 

d.  Aggregates. 

Under  the  term  aggregates  are  here  included  only  those  mineral 
aggregations  which  are  homogeneous,  or  which  cannot  be  shown  to 
be  heterogeneous.  They  may  consist  of  amorphous  or  of  crystalline 
substances ;  but  since  their  chief  characteristic  is  their  optical  behavior^ 
they  cannot  be  properly  described  before  the  optical  properties  of 
minerals  in  thin  section  have  been  discussed.  They  will  therefore 
be  considered  at  the  end  of  the  chapter  on  that  subject. 


VLEAVAGE.  21 


PHYSICAL  PROPERTIES. 

AMONG  the  physical  properties  of  minerals  their  cohesion  and 
behavior  towards  light  are  specially  useful  in  microscopical  studies. 

I.  PHENOMENA   OF  COHESION. 

Cleavage. — Through  the  shattering  consequent  upon  grinding,  cracks 
and  crevices  are  formed  in  many  minerals,  the  sharpness  and  more  or 
less  continuous  course  of  which  depends  on  the  greater  or  less  perfec- 
tion of  the  cleavage  in  the  particular  mineral,  while  their  direction  cor- 
responds to  the  intersection  of  the  cleavage  planes  with  that  of  the 
section. 

Cohen  states  that  by  heating  thin  sections  to  redness  cleavage  cracks 
sometimes  arise,  which  did  not  make  their  appearance  during  the 
grinding.  The  more  perfect  the  cleavage  of  a  mineral  is,  the  more 
closely  crowded,  uninterrupted,  and  sharp  will  be  the  cleavage  cracks 
in  its  thin  section.  In  less  perfectly  cleavable  substances  the  cleavage 
cracks  are  less  frequent,  and  it  is  highly  characteristic  of  some  minerals 
that  the  cracks  often  stop  in  the  middle  of  a  section  and  reappear  in 
another  parallel  plane,  while  an  irregular  fissure  connects  the  two 
cleavage  cracks.  The  perfection  of  the  cleavage  cracks  depends  also 
on  the  angle  at  which  the  section  cuts  the  plane  of  cleavage.  They  are 
sharpest  when  these  directions  are  at  right  angles  to  one  another.  In 
an  inclined  position  the  cleavage  cracks  often  appear  broad  and  dark, 
because  of  the  total  reflection  from  the  capillary  layer  of  air  between 
their  walls ;  their  margins  are  sometimes  very  finely  indented. 

Cleavage  cracks,  especially  in  colorless  minerals,  may  often  remain 
undetected  by  full  illumination, — that  is,  in  a  strong  light, — and  first 
become  evident  with  dull  illumination,  which  is  obtained  by  depress- 
ing the  polarizer  and  its  lens,  or  the  condenser  when  using  strongly 
convergent  light. 

The  fracture  of  a  mineral  has  no  corresponding  microscopical 
.  phenomenon :  the  irregular  cracks  and  fissures  in  cleavable  and  un- 
cleavable  minerals  either  result  from  aggregation,  following  the  out- 
lines of  the  individuals  composing  the  aggregate,  or  correspond  to 


22  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

previously  existing  internal  fractures,  which  in  many  cases  have  been 
developed  by  mechanical  pressure  while  the  minerals  were  part  of  a 
mountain  mass ;  in  other  cases  they  have  been  produced  by  chemical 
processes,  for  example,  the  cracking  of  olivine  through  serpentiniza- 
tion,  etc. 

The  course  and  the  relative  position  of  cleavage  cracks  depend  on 
the  direction  in  which  the  section  cuts  the  mineral.  A  pyramidal 
cleavage  always  furnishes  four  systems  of  parallel  cleavage  cracks, 
which  intersect  at  angles  dependent  on  the  position  of  the  section.  A 
good  example  is  anatase.  An  exception  to  this  rule  would  occur  in 
the  case  of  cleavage  after  a  holohedral  hexagonal  pyramid,  which  is  not 
met  with  among  the  petrographical  minerals.  Prismatic  cleavage 
furnishes  two  (in  the  hexagonal  system  three)  systems  of  parallel  lines, 
which  cross  one  another  so  long  as  the  section  lies  at  right  angles  or 
inclined  to  the  axis  of  the  prism,  but  which  are  all  parallel  when  the 
section  is  parallel  to  this  axis.  Pinacoidal  cleavage,  the  cleavage  face 
being  parallel  to  two  axes,  gives  parallel  lines  in  all  sections,  except  in 
the  case  of  the  regular  cube  (the  isometric  system). 

Cleavage  parallel  to  several  pinacoids  would  produce  the  same 
effect  as  prismatic  or  pyramidal  cleavage,  but  would  be  distinguished 
from  these  by  the  unequal  perfection  of  the  cleavage  cracks  parallel  to 
the  different  pinacoids.  PL  IX.  Figs.  5  and  6,  PL  X.  Figs.  1-6,  and 
PL  XI.  Figs.  1-3  present  different  degrees  of  perfection  and  the 
mutual  position  of  cleavage  cracks. 

It  is  evident  that  one  can  calculate  the  angle  at  which  the  cleavage 
planes  must  intersect  when  the  position  of  the  section  and  the  normal 
cleavage  angle  are  known.  In  the  same  manner,  from  the  cleavage 
angle  measured  for  a  particular  case  the  position  of  the  section  may  be 
determined  when  the  zone  in  which  it  lies  is  known,  which  is  often 
recognized  with  approximate  certainty  by  optical  means. 

Gliding  planes  (Gleitfliichen)  &n&  pressure  planes  (Druckflachen) 
also  give  rise  to  cracks  which  from  their  appearance  cannot  be  distin- 
guished from  cleavage  cracks.  Up  to  the  present  they  have  been 
recognized  only  in  mica  and  cyanite  (PL  IY.  Fig.  5)  ;  but  it  may  be 
stated  that  they  have  a  wider  distribution,  and  that  certain  planes  of 
parting  (Absonderungsflachen)  observed  in  the  pyroxene,  amphibole, 
and  feldspar  groups  may  be  considered  as  pressure  planes. 

The  investigation  of  chemical  cohesion  by  means  of  etched  figures 
in  petrographical  work  is  somewhat  hindered  by  the  dependence  of  the 
form  of  the  etched  figures  on  the  position  of  the  surface  etched,  and 
by  the  uncertain  determination  of  the  position  of  this  plane  in  thin 


TRANSMISSION  OF  LIGHT.  23 

sections.  Nevertheless  it  is  serviceable  in  particular  cases,  which  will 
be  mentioned  under  the  description  of  the  chemical  properties  and  in 
the  second  part  of  this  book. 


II.  OPTICAL  PROPERTIES. 

a.  Refraction  and  Index  of  Refraction  in  Isotropic  Media. 

Literature. 

BABINET,  Ueber  die  optischen  Kennzeicken  der  Mineralien.     Comptes  rendus  1837. 

I.  758  und  Auszug  in  Pogg.  Ann.  XLI   115.  1837. 
A.  DES  CLOIZEAUX,  M&noire  sur  1'emploi  du  microscope  polarisant  et  sur  Petude 

des  proprietes  optiques  birefringentes  propres  a  determiner  le  systeine  cristallin 

des  cristaux  naturels  ou  artificiels.  Ann.  des  Mines  VI.    1864  and  Pogg.  Ann. 

CXXVL  1865. 

—  De  1'emploi  des  proprietes  optiques  birefringentes  en  inineralogie.  Paris.  1857. 

—  Nouvelles  recherches  sur  les  proprietes  optiques  des  cristaux  naturels  ou  artificiels 

et  sur  les  variations  que  ces  proprietes  eprouvent  sous  I'influence  de  la  chaleur. 
Mem.  pres.  a  1'Institut  imperial  de  France.  T.  XVIII.  1867. 

Optics  teaches  us  that  light  is  transmitted  in  a  straight  line  without 
change  of  direction  in  one  and  the  same  homogeneous  medium  as  vibra- 
tions of  particles  of  the  luminiferous  ether,  which  take  place  at  right 
angles  to  the  direction  of  transmission.  There  are  media  in  which  the 
velocity  of  transmission  of  the  light  is  independent  of  the  direction  in 
which  it  is  propagated :  these  are  called  isotropic.  In  other  media 
the  velocity  of  transmission  changes  with  the  direction  :  these  are  called 
anisotropic. 

Gaseous,  fluid,  and  amorphous  (glassy)  bodies,  and  those  crystalliz- 
ing in  the  regular  system,  are  isotropic  ;  on  the  other  hand,- substances 
crystallizing  in  the  quadratic,  hexagonal,  orthorhombic,  monoclinic, 
and  triclinic  systems  are  anisotropic. 

The  independence  of  the  rate  of  transmission  of  light  of  its  direc- 
tion in  isotropic  media  leads  to  the  conclusion  that  the  distribution 
and  elasticity  of  the  luminiferous  ether  is  the  same  in  all  directions 
throughout  such  media.  The  absolute  magnitude  of  the  elasticity  of 
this  ether  in  materially  different  media  is  different,  and  since  the  rate 
of  transmission  (velocity)  of  the  light  is  proportional  to  the  square  root 
of  the  elasticity  of  the  ether,  light  will  be  transmitted  in  different  iso* 
tropic,  homogeneous  media  at  different  rates  (with  different  velocities). 
Thus  there  are  optically  denser  and  optically  rarer  media. 


24  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

In  consequence  of  these  different  optical  densities,  a  ray  of  light  is 
generally  diverted  from  its  former  direction  in  passing  out  of  one 
medium  into  another  and  always  experience/I  a  change  in  its  rate  of 
transmission.  This  is  termed  the  refraction  of  light. 

The  phenomena  connected  with  the  passage  of  a  ray  of  light  out 
of  one  homogeneous  isotropic  medium  into  another  homogeneous  iso- 
tropic medium  of  different  optical  density  are  the  following  (air 

being  taken  as  the  medium  out  of 
which  the  ray  of  light  comes)  :  Let 
ab  be  the  bounding  plane  between  the 
air  and  the  second  isotropic  body 
(Fig.  2),/b  the  incident  ray  of  light, 
and  de  the  normal  to  ab  at  the  point 
of  incidence  c.  Then  will  the  light 
reaching  c  from  the  direction  fc  in 
part  pass  into  the  second  medium  in  a 
changed  direction  and  with  different 
velocity  (the  refracted  ray),  part  of  it 
will  return  in  the  first  medium  ac- 
cording to  a  definite  law  (the  reflected 
ray),  and  part  will  be  scattered  irregularly  in  all  directions  or  be  diffused. 
Most  of  this  diffusion  of  the  light  would  not  take  place  if  ab  were  a 
mathematical  plane,  since  it  arises  from  an  unevenness  of  the  surface. 
If  one  calls  the  angle  which  the  incident  (cf),  the  reflected  (eg),  and  the 
refracted  ray  (he)  make  with  the  normal  ecd,  the  angle  of  incidence  =  ?', 
angle  of  reflection  =  r,  and  angle  of  refraction  =  p ;  and  further,  the 
planes  through  each,  of  these  rays  and  the  normal,  the  plane  of  in- 
cidence, plane  of  reflection,  and  plane  of  refraction,  respectively,  then 
there  exists  between  these  quantities  the  following  relations  : 

(1)  The  planes   of    incidence,  reflection,    and   refraction   fall    to- 
gether. 

(2)  The  angle   of  incidence   is   equal  to  the  angle  of  reflection  ; 


(3)  The  angle  of  incidence  and  angle  of  refraction  bear  a  constant 
relation  to  one  another. 

Describing  about  c  a  circle  with  cf  as  radius  arid  letting  fall  from 
/"and  h  (fk  and  hi)  perpendicular  to  the  normal  dee,  then  sin  i=Jcf 
and  sin  p  =  hi.  Then  whatever  be  the  direction  of  the  incident  .ray, 
that  of  the  corresponding  refracted  ray  (the  media  remaining  the  same) 
is  so  conditioned  that  the  quotient  of  the  sine  of  the  angle  of  refraction 
into  the  sine  of  the  angle  of  incidence  is  a  constant  quantity  (n  or  /*), 


DISPERSION  OF  LIGHT.  25 

which  is  called  the  index  of  refraction  or  coefficient  of  refraction. 
Thus  the  third  relation  may  be  precisely  formulated  : 

sin  i 

-. —  =n. 

smp 

The  index  of  refraction  n  is  therefore  a  constant,  which  can  be  em- 
ployed in  the  determination  of  a  substance,  just  as  the  specific  gravity 
or  any  other  constant.  By  the  term  "  index  of  refraction,"  as  ordinarily 
used,  is  understood  the  index  of  refraction  of  an  isotropic  medium 
compared  with  air ;  and  since  the  index  of  refraction  of  air  compared 
with  a  vacuum  varies  with  the  thermometer  and  barometer,  it  is  de- 
pendent on  temperature  and  pressure ;  at  760  mm.  pressure  and  0°  C. 
temperature,  n  =  1.000294. 

The  index  of  refraction  of  isotropic  media  compared  with  a  vacuum 
is  called  their  absolute  index  of  refraction.  In  actual  practice  and 
tinder  all  the  conditions  of  pressure  and  temperature  found  at  the  sur- 
face of  the  earth  the  index  of  refraction  may  be  considered  unchange- 
able. This  index  of  refraction  for  almost  all  fluid  and  isotropic  solid 
media  lies  between  1  and  2,  and  seldom  exceeds  the  latter  figure.  For 
example,  it  is  1.336  for  water,  1.498  for  rock  salt,  1.553  for  glass, 
1.435  for  fluorite,  2.270  for  diamond.  In  passing  into  an  optically 
denser  medium  the  incident  ray  is  bent  toward  the  perpendicular,  when 
into  an  optically  rarer  medium  it  is  bent  from  the  perpendicular. 

Finally,  the  amount  of  deflection  upon  the  passage  of  a  ray  from  air 
into  another  medium  is  dependent  on  the  wave-length  of  the  incident 
ray;  it  is  consequently  different  for  different-colored  rays,  and  is  in- 
versely proportioned  to  the  wave-length.  Thus  the  index  of  refraction 
for  blue  rays  is  greater  than  for  red,  nv  >  np. 

This  phenomenon  is  called  the  dispersion  of  light.     Its  amount  is 

different  for  different  media,  andvis  measured  by  -*-.     Moreover,  the 

MV 

amount  of  difference  in  the  dispersion  for  particular  colored  rays — 
yellow  and  green,  for  instance — holds  no  general*  relation  to  the  total 
dispersion,  but  is  different  and  characteristic  for  each  part  of  the  spec- 
trum in  each  and  every  substance. 

From  the  ratio  —  —  =  n,  when  n  and  i  are  known,  the  direction  of 
sinp 

the  refracted  ray  may  be  calculated.  Among  all  possible  values  for 
?V there  are  three  of  special  importance,  namely,?'  —  0°,  i  =  90°,  and 

tan  /  =  n. 


2(>  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

If  i  =  0°,  the  incident  ray  coincides  with  the  perpendicular,  and 
=  n ;  that  is,  the  angle  of  refraction  =0°,  and  the  transmitted  ray 


sin  p 

coincides  with  the  perpendicular.  Thus  when  the  incident  ray  is  per- 
pendicular to  the  bounding  plane  there  is  no  deflection  of  the  trans- 
mitted ray,  only  a  change  in  its  rate  of  propagation. 

If  i  =  90°  (grazing  incidence),  — =  n  or  sin  p  =  —.      This  value 

sin  p  n 

of  the  angle  of  refraction  is  called  the  limiting  or  critical  angle  •  for 
water  this  is  48°  35',  for  flint  glass  37°  36',  for  diamond  23°  53'. 
From  the  general  Jaw  that  a  motion  follows  the  same  way  back  as 
forth,  a  ray  of  light  from  a  denser  medium  coming  upon  a  rarer 
medium  at  the  critical  angle  continues  parallel  to  the  bounding  plane 
between  both  media,  that  is,  at  right  angles  to  the  normal. 

If  the  light  from  a  denser  medium  strikes  a  rarer  one  at  a  greater 
angle  than  the  limiting  angle  it  cannot  pass  into  the  latter,  but  will  be 
reflected  from  the  bounding  plane.  Since,  in  distinction  to  the  pre- 
viously mentioned  reflection,  no  part  of  the  light  in  this  case  enters  the 
second  medium,  this  latter  reflection  is  called  total  reflection.  This 
cannot  take  place  on  the  passage  of  light  from  a  rarer  into  a  denser 
medium. 

The  above-mentioned  relations  explain  certain  phenomena  which 
are  very  frequently  observed  in  the  microscopical  investigation  of 
minerals  and  rocks.  If  we  imagine  any  particular  substance  enclosed 
in  another  of  exactly  the  same  color  and  index  of  refraction,  then  the 
boundaries  of  the  enclosed  substance  against  the  surrounding  one 
could  not  be  observed  at  all.  On  the  other  hand,  the  enclosed  sub- 
stance would  have  the  highest  degree  of  transparency  in  all  its  parts. 
Therefore  if  it  is  desirable  to  see  the  outward  form  of  a  substance  with 
the  greatest  possible  sharpness  it  must  be  immersed  in  a  medium  with 
as  different  an  index  of  refraction  as  possible  (air  or  water).  But  if  it 
is  desired  to  observe  particularly  the  internal  characters  of  the  sub- 
stance, an  envelope  with  as  nearly  the  same  index  of  refraction  as 
possible  should  be  chosen  (oil  and  other  strongly  refracting  fluids,  or 
Canada  balsam).  If  substances  with  various  indices  of  refraction 
immersed  in  the  same  envelope  of  water,  oil,  or  solid  are  studied 
simultaneously,  the  surface  of  one  appears  smooth  and  even,  while  that 
of  another  is  rough  and  wrinkled.  The  latter  are  said  to  be  shagreened. 
The  surface  of  that  substance  will  appear  smooth  whose  index  of  re- 
fraction is  smaller  or  equal  to  that  of  the  envelope,  for  all  of  the  rays 
coming  out  of  it  can  pass  through  the  surrounding  substance.  If, 


INDEX  OF  REFRACTION.  27 

however,  the  enclosed  body  is  more  strongly  refracting  than  its 
envelope,  there  will  be  many  rays  which  will  strike  the  rough  surface, 
produced  by  incomplete  polishing  of  the  section,  at  angles  greater  than 
the  limiting  angle,  and  these  will  suffer  total  reflection,  in  consequence 
of  which  the  surface  of  the  substance  is  visible  because  of  a  diminu- 
tion of  the  light.  One  and  the  same  substance  will  therefore  show  a 
smooth  surface  in  certain  envelopes  and  a  rough  one  in  others,  so  when: 
the  index  of  refraction  of  the  enclos- 
ing substance  is  known  the  refraction 
of  the  enclosed  substance  may  be 
inferred. 

Strongly  refracting  minerals  ap- 
pear more  glaring  or  clearer  in  con- 
trast to  less  refracting  ones,  because 
the  amount  of  light  striking  any 
point  of  the  former  becomes  con- 
centrated into  a  smaller  part  of  the 

surface.  If  there  falls  on  the  point  r  of  the  lamella  ABCD  (Fig.  3)  a 
hemispherical  bundle  of  rays  mon,  the  same  become  within  the  lamella  a 
cone  of  rays  srt>  the  radius  of  whose  base  j?£  has  the  following  relations : 

sin  i 

—  —  n. 

.    x 

6111  a 

X         1 

For  ^  =r  90°,  sin-=-.     The  circular  base,  therefore,  is   smaller  in 
2t      it 

proportion  as  the  index  of  refraction  of  the  lamella  is  larger,  and  con- 
sequently the  illumination  becomes  stronger,  in  fact,  in  proportion  to 
the  squares  of  the  indices.  On  the  other  hand,  the  boundary  of  the 
more  strongly  refracting  body  against  the  less  refracting  must  appear 
the  darker  in  proportion  as  the  difference  between  their  indices  of  re- 
fraction is  greater,  because  the  critical  angle  becomes  the  smaller  and 
the  total  reflection  occurs  so  much  the  sooner. 

For  this  reason  gases  enclosed  in  solid  or  liquid  bodies  have  very 
broad  total  reflection  borders,  while  those  borders  for  fluid-inclusions, 
cceteris  paribus,  are  smaller,  and  for  inclusions  of  solids  within  solids 
still  smaller.  These  relations  are  made  use  of  in  distinguishing  gas- 
eous, fluid,  and  solid  inclusions  in  minerals  from  one  another,  and  the 
breadth  of  the  total  reflection  border  of  gas  bubbles  compared  with  the 
size  of  the  clear  centre  of  the  same  may  be  employed  to  determine  the 
size  of  the  index  of  refraction  of  the  enclosing  substance. 


28  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

Among  the  various  microscopical  methods  used  for  determining 
the  index  of  refraction  of  isotropic  media  is  the  following:  If  one 
focuses  the  objective  of  a  microscope  exactly  on  any  point,  and  then 
slips  between  this  and  the  objective  a  refracting  medium, — for  example, 
a  glass  plate  with  parallel  faces, — then  the  object  which  was  distinctly 
seen  before  is  no  longer  visible,  or  not  distinctly  so,  and  the  objective 

of  the  microscope  must  be  raised 
a  certain  amount  in  order  to  see 
the  point  as  distinctly  as  before. 
The  extent  to  which  the  point  in 
question  appears  to  be  raised  de- 
pends on  tUe  thickness  of  the  in- 
serted plate  and  its  index  of 
refraction.  'Let  o  (Fig.  4)  be  the 
point  in  air  ;  if  the  lamella  L  be 
placed  over  it,  then  a  ray  oba  will  reach  the  objective  through  the 
lamella  with  unaltered  direction,  but  with  altered  velocity.  A  ray  <?<?,  on 
the  other  hand,  will  be  deflected  at  c  to  eg,  since  it  passes  into  air,  and 
will  appear  to  come  from  r.  In  r,  where  both  rays  intersect,  the  point 
o  will  apear  to  be;  it  is  therefore  raised  a  distance  TO.  Placing  ro  — 
h,  ob  —  2),  we  have,  if  dfis  the  perpendicular  at  c  and  rq  is  at  right 
angles  to  df, 


.  4 


.       cb       T  .  or          cb 

tan  ^  —  ^  and  tan  p  =  +-  —  -=- — —  ; 
D  qc       D  —h. ' 


therefore, 


tan  «  _  D  —  h 
tanp          D 


And  since  for  small  angles,  which  are  those  met  with  in  microscopical 
observation  the  tangents  and  sines  may  be  interchanged,  this  equation 
becomes 


sm 


sn  p 


D  —  h 
D 


for  the  passage  of  light  from  the  lamella  into  air.     For  the  passage  of 
light  from  air  into  the  lamella,  i  and  p  are  reversed,  and  we  have 


n  = 


D 

D-h 


INDEX  OF  REFRACTION.  29 

The  process  of  measurement  may  be  varied  in  a  great  variety  of  ways 
for  particular  cases.  It  must  not  be  forgotten  that  the  accuracy  of  the 
process  increases  with  the  closeness  of  the  focusing :  the  sharpest  possi- 
ble test-objects  should  therefore  be  chosen,  and  the  highest  practicable 
magnifying  power.  The  weak  point  of  the  method  is  the  determina- 
tion of  the  thickness  of  the  lamella,  because  this  is  seldom  the  same  for 
all  parts  of  the  lamella,  and  should  be  determined  at  the  point  where 
h  is  to  be  measured. 

To  determine  the  index  of  refraction  in  thin  sections  where  the 
lamellae  lie  between  Canada  balsam  and  glass,  it  is  best  to  make  use  of 
the  law  readily  derived  from  the  foregoing,  that  the  apparent  thicknesses- 

of  two  equally  thick  lamellae  are  inversely  proportional  to  their  indices 

jy 
of  refraction,  n1  :  n  —  D'  :  DJ,  and  nl  =  n.  -=—.     There  is  placed  on 

the  same  glass,  alongside  of  the  thin  section  of  the  substance  to  be 
investigated,  a  thin  section  of  the  same  thickness  of  a  substance  whose 
coefficient  of  refraction  (n)  is  known,  or  another  known  substance  in 
the  first  thin  section  may  be  used,  or  finally  the  Canada  balsam  itself, 
if  its  coefficient  be  known.  The  apparent  thickness  Z>/  of  the  sub- 
stance in  question  is  measured,  and  also  the  difference,  d,  between  the 
focusing  on  the  test-object  seen  through  the  lamella  under  investiga- 
tion, and  through  the  known  lamella.  And  since  this  difference  may 
be  positive  or  negative  according  as  the  first  lamella  is  more  strongly 
or  more  weakly  refracting  than  the  known  lamella,  we  have 


in  which  nl  alone  is  unknown. 

In  spite  of  the  fact  that  the  apparent  thickness  of  a  lamella  is 
smaller  the  larger  its  index  of  refraction,  strbngly  refracting  substances 
in  rock  sections  stand  out  in  relief  from  the  web  of  less  refracting 
substances  around  them,  and  one  can  judge  after  a  little  practice  of 
the  relative  indices  of  refraction  of  any  two  substances  from  their 
greater  or  less  relief  as  compared  with  one  another.  This  apparently 
contradictory  phenomenon  is  the  result  of  several  circumstances.  The 
more  glaring  illumination  of  the  surface  of  strongly  refracting  lamellae 
combined  with  the  marginal  total  reflection  causes  their  surface  to 
appear  nearer  than  that  of  less  glaring  lamellae ;  moreover,  the  fact  that 
their  under  surface  appears  more  raised  combined  with  the  conscious- 
ness that  both  are  of  equal  thickness  increases  the  impression  that  the 
upper  surface  projects  in  relief. 


30 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


Through  the  refraction  of  light  upon  its  passage  out  of  one 
isotropic  medium  into  another,  not  only  its  direction  and  rate  of  trans- 
mission are  changed,  but  another  phenomenon  shows  itself  to  a  greater 
or  less  degree,  which  is  called  the  polarization  of  light.  Since  light 
is  propagated  as  vibrations  of  ether  particles  at  right  angles  to  its  di- 
rection, and  since  there  are  innumerable  normals  to  a  line  in  space,  the 
particles  of  ether  may  vibrate  in  an  endless  number  of  planes  during 
the  propagation  of  ordinary  light.  On  the  other  hand,  polarized  light 
is  that  which  is  propagated  as  vibrations  of  the  ether  in  a  single  plane. 
The  difference  between  these  two  kinds  of  light  cannot  be  detected 
by  the  unaided  eye.  Polarized  light  may  be  recognized  by  its  being 
in  some  cases  completely  absorbed  by  doubly-refracting,  absorbing 
media,  by  its  not  being  reflected  from  mirrors  under  certain  condi- 
tions, nor  resolved  (analyzed)  by  doubly-refracting  media  under  par- 
ticular conditions. 

The  reflection  and  refraction  in  isotropic  media  are  among  the 
many  processes  through  which  ordinary  light  becomes  polarized  ;  that 
is,  has  all  the  oscillations  of  the  vibrating  luminiferous  ether  reduced 
to  one  azimuth. 

A  partial  polarization  of  light 
takes  place  with  every  reflection  and 
refraction.  But  when  the  reflected  ray 
stands  at  right  angles  to  the  refracted 
one,  both  of  these  rays  are  polarized,  the 
reflected  one  is  completely  polarized 
when  the  substance  is  transparent,  and 
the  planes  of  polarization  are  perpen- 
dicular to  one  another. 

The  angle  of  incidence  for   which 
the  reflected  and  refracted  rays  are  at 
right  angles  to  one  another  is  one  pe- 
culiar  to  every  substance,  and  naturally 
depends  on  its  index  of  refraction. 

Let  AC  (Fig.  5)  be  the  incident  ray  and  ¥.BCF=W°.     Then 

/?/T 

tan  i  =  tan  r  —  -^77;  since  BK—  sin  r  —  sin  i  and  KC  =  FG  =  sin  p, 


,,  .      sn 

then  tan  ^  =  -  —  =  n. 
sin  p 


Therefore  the  reflected  and  refracted  rays  are  polarized  at  right 
angles  to  one  another  when  the  tangent  of  the  angle  of  incidence  is 
equal  to  the  index  of  refraction.  This  angle  is  called  the  polarization 


DOUBLE  REFRACTION.  31 

aiKjlc.  It  is  assumed  that  the  plane  in  which  the  vibrations  of  the  re- 
flected ray  take  place  stands  at  right  angles  to  the  plane  of  reflection, 
also  called  the  plane  of  polarization  ;  then  the  plane  of  vibration  of  the 
refracted  ray  is  the  same  as  the  plane  of  reflection. 


Double  Refraction  in  Anisotropic  Media.4  t 


If  one  imagines  a  luminous  movement  to  take  place  from  "any 
point  within  an  isotropic  medium,  then  this  will  advance  (be  trans- 
mitted) in  all  directions  with  the  same  velocity,  and  the  wave-surface 
at  any  moment  will  be  the  surface  of  a  sphere  whose  radius  is  ptopor- 
tional  to  the  time  which  has  elapsed  since  the  beginning  of  the  move- 
ment. But  if  the  luminous  movement  starts  from  a  point  within  an 
anisotropic  medium,  in  which  the  velocity  of  transmission  varies  with 
the 'direction,  it  will  advance  in  different  directions  with  different 
velocities  ;  and  the  wave-surface  can  no  longer  be  a  sphere,  but  will  be 
a  warped  surface,  whose  form  ana  position  stands  in.  the  closest  con- 
nection with  the  molecular  structure  of  the  anisotropic  medium. 

If  now  a  ray  of  ordinary  light  from  air,,  an  isotropic  medium,  falls 
on  an  anisotropic  medium  and  penetrates  it,  since  there  are  in  the  ani- 
sotropic medium  different  elasticities  corresponding  to  all  the  possible 
azimuths  in  which  the  vibrations  of  the  ray  of  ordinary  light  take 
place,  then,  for  perpendicular  incidence,  these  vibrations  will  be  re- 
duced to  the  two  azimuths,  which  correspond  to  the  directions  of 
greatest  and  least  elasticity  lying  at  right  angles  to  the  direction  of 
transmission  of  the  ray.  There  arise  therefore  out  of  the  incident  ray 
two  rays,  which  in  this  particular  case  are  transmitted  in  approximately 
the  same  direction,  with  oscillations  perpendicular  to  one  another  and 
with  different  velocities,  since  their  vibrations  correspond  to  different 
elasticities.  Both  rays  are  polarized  because  their  vibrations  in  each 
case  lie  in  one  azimuth,  and  they  are  polarized  at  right  angles  to  one 
another,  because  the  directions  of  greatest  and  least  elasticity  cannot  be 
other  than  at  right  angles  to  one  another  within  the  plane  normal  to  the 
direction  of  the  transmission  of  the  incident  ray.  If  both  these  rays 
emerge  into  air  through  a  surface  parallel  to  that  through  which  they 
entered,  no  deflection  will  take  place;  but  if  the  face  of  exit  is  in- 
clined to  that  of  entrance,  the  two  rays  which  reach  the  exit  face  with 
different  velocities  pass  into  the  air  at  different  angles. 

If  the  incident  ray  coming  through  air  strikes  the  surface  of  an 
anisotropic  medium  not  perpendicularly,  but  obliquely,  the  two  rays 
obeying  the  laws  of  elasticity  in  the  anisotropic  medium  will  traverse 


PHYSIOGRAPHY  OF  THE  ROCK- MAKING  MINERALS. 

it  in  different  directions.  The  incident  raj  will  therefore  be  separated 
into  two  different  rays  deflected  or  refracted  to  different  degrees,  and 
for  this  reason  anisotropic  media  are  also  called  doubly-refracting. 

All  crystalline  bodies,  not  belonging  to  the  isometric  system,  are 
anisotropic  or  doubly-refracting  media,  and  therefore  possess  the  com- 
mon property  of  generally  separating  an  incident  ray  into  two,  which 
traverse  these  bodies  with  different  velocities,  in  different  directions, 
and  with  planes  of  vibration  or  of  polarization  at  right  angles  to  one 
another.  The  characteristics  and  phenomena  connected  with  the  dis- 
tribution of  the  elasticity  of  the  ether  vary  according  to  whether  a 
<T)^Mj  belongs  to  a  system  with  a  principal  axis  (tetragonal  and  hexag- 
onal^)!1 to  one  without  a  principal  axis  (orthorhornbic,  monoclinic, 
triclinic). 


Double  Refraction  in  Crystals  with  a  Principal  Axis. . 

If  one  assumes — and  this  assumption  explains  the  dioptric  phe- 
nomena of  uniaxial  crystals — that  the  distribution  of  the  particles  of 
ether,  as  wfcll  as  those  of  the  mass,  is  symmetrical  with  respect  to  the 
principal  axis,  it  follows  that  at  right  angles  to  this  axis  the  disposi- 
tion and  consequent  elasticity  of  the  ether  must  be  the  same  in  all 
directions,  and  that  in  a  direction  parallel  to  the  principal  axis  the 
elasticity  must  have  a  maximum  difference  from  this ;  moreover,  the 
elasticity  in  a  direction  which  makes  an  angle  <p  <  90°  with  the  princi- 
pal axis  must  be  intermediate  between  the  first  two,  its  amount  de- 
pending on  the  angle  0,  and  it  must  be  the  same  for  all  directions 
which  have  the  same  inclination  to  the  principal  axis. 

From  this  it  follows  that  if  the  square  root  of  the  elasticity  of  the 
ether  at  right  angles  to  the  principal  axis  and  parallel  to  it  be  repre- 
sented by  two  bisecting  lines  normal  to  one  another,  and  a  circle  be 
described  with  radius  equal  to  half  the  square  root  of  the  elasticity  at 
right  angles  to  the  principal  axis  and  an  ellipse  be  formed  about  the 
two  bisecting  lines, — that  is,  one  whose  diameters  correspond  to  the 
square  root  of  the  greatest  and  least  elasticity, — and  this  be  rotated 
about  the  diameter  corresponding  to  the  square  root  of  the  elasticity 
parallel  to  the  principal  axis,  the  resulting  ellipsoid  of  rotation  will 
represent  the  distribution  of  the  elasticity  of  the  ether  in  the  crystal. 
This  ellipsoid  of  rotation  is  also  called  the  ellipsoid  of  elasticity. 

Both  the  velocity  and  direction  of  vibration  of  the  rays  produced 
by  the  double  refraction  in  a  uniaxial  crystal  are  obtained  oy  passing 
a  plane  through  the  centre  of  the  ellipsoid  at  right  angles  to  the  inci- 
dent ray.  The  vibrations  of  both  rays  take  place  parallel  to  the  g 


DOUBLE  REFRACTION. 

est  and  least  diameters  of  this  cross-section,  and  the  lengths  of  thes 
two  diameters  express  the  velocity  of  the  rays. 

Now  the  section  through  the  ellipsoid  at  right  angles  to  the  princ 
pal  axis  (or  axis  of  rotation)  is  a  circle,  and  in  this  all  the  diameters  ai 
equal.     Kays,  then,  which  enter  the  crystal   parallel  to  the  prim 
axis  suffer  neither  refraction  nor  polarization,  but  the  light  travc 
the  crystal  in  this  case  just  as  it  would  through  an  isotropic  median 
The  principal  crystallographic  axis  is  consequently  a  direction  of  *h 
pie  refraction,  and  is  for  this  reason  called  the  optic  axis.     Thus  It  i 
optically  and  morphologically  a  singular  axis. 

Every  other  section  through  the  ellipsoid  would  be  an  ellipsj  , 
which  would  be  the  more  elongated  the  greater  the  angle  made  by  tfjH 
incident  ray  and  the  principal  axis,  and  which  would  approach  a  ciivl 
as  this  angle  diminished.  In  all  these  ellipses  one  diameter  (the  equi. 
torial)  remains  the  same,,  and  is  equal  to  the  diameter  of  the  circula 
section.  That  one  of  the  two  refracted  rays  which  vibrates  parallel  t 
this  diameter  advances  with  a  velocity  which  is  independent  of  th 
direction  of  transmission,  and  is  the  same  in  all  directions;  it  behave 
like  a  ray  in  an  isotropic  medium,  except  for  its  polarization,  and  ha 
a  constant  index  of  refraction.  It  is  called  the  ordinary  ray  ( O ) ;  i 
the  plane  passing  through  the  incident  ray  and  the  principal  axis  (opti 
axis)  is  called  the  principal  optic  section  (optische  Hauptschnitt\  the? 
the  vibrations  of  the  ordinary  ray  lie  perpendicular  to  th^  principal 
optic  section. 

The  second  diameter  of  every  possible  section  naturally  lies  in  th 
principal  optic  section,  its  length  is  dependent  on  the  inclination  < 
the  section  to  the  principal  axis,  and  is   therefore  variable.     The  ra 
vibrating  parallel  to  this  diameter — that  is,  in  the  principal  optic 
tion — will  therefore  traverse  the  crystal  with  a  velocity  varying  wit 
the  angle  of  incidence,  consequently  it  has  no  constant  coefficient  c. 
refraction.    This  latter,  in  fact,  must  vary  the  more  with  the  dirertioi 
since  it  is  inversely  proportional   to  the  velocity  of  transmission  ; 
approaches  the  value  of  the  index  of  refraction  of  the  ordinary  ray  < 
the  angle  between  the  incident  ray  and  the  principal  axis  approach* 
zero,  and  reaches  a  maximum  when  this  angle  becomes  90°.     This  ra 
which  vibrates  in  the  principal  optic  plane  is  called  the  extr<iordi> 
ray  (-E)-     When  the  index  of  refraction  of  the  extraordinary  ray 
spoken  of,  it  is  understood  to  be,  the  index  of  refraction  for  incid- 
perpendicular  to  the  principal  axis,  and  is  designated  by  the  letter  > 
while  the  index  of  refraction  of  the  ordinary  ray  is  &>.     Natural 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

and  e  are  dependent  on  the  wave-length,  and  therefore  change  with 
the  color  of  the  light,  as  n  does  for  isotropic  media. 

As  the  principal  crystallographic  axis  may  be  longer  or  shorter 
than  one  of  the  secondary  axes,  so  the  elasticity  in  the  direction  of  the 
primary  axis  may  be  greater  or  smaller  than  at  right  angles  to  it.  If 
the  primary  axis  is  the  direction  of  greatest  elasticity,  the  crystal  is 
said  to  be  optically  negative  or  repulsive,  and  the  extraordinary  ray  is 
less  strongly  refracted  than  the  ordinary  ray  (GO  >  e)  and  advances 
with  greater  velocity.  Optically  positive  or  attractive  crystals  are 
those  for  which  the  reverse  relation  holds  ;  for  these,  then,  GO  <  e. 

The  optical  character  »of  crystals  with  a  principal  axis  (tetragonal 
d  hexagonal)  which  distinguishes  them  from  those  of  the  isometric 
system  and  from  amorphous  bodies  is  their  double  refraction,  and  that 
ivhich  distinguishes  them  from  crystals  of  the  remaining  systems  is 
the  presence  of  a  single  optic  axis  coincident  with  the  primary  axis. 
Tetragonal  and  hexagonal  crystals  are  collectively  called  optically  uni- 

•  l  crystals.  The  optic  axis  is  characterized  by  the  fact  that  all  rays 
propagated  parallel  to  it  traverse  the  crystal  with  the  same  velocity ; 
that  all  rays  vibrating  at  right  angles  to  it  advance  with  equal  veloci- 
ties in  all  directions  ;  and  finally,  that  every  plane  passing  through  the 
optic  axis  is  a  plane  of  symmetry  for  the  ellipsoid  of  elasticity. 

Double  Refraction  in  Crystals  without  a  Primary  Axis. 

The  phenomena  connected  with  the  transmission  of  light  through 
a  crystal  of  the  orthorhombic,  monoclinic,  or  triclinic  systems  show 
that  the  distribution  of  the  elasticity  of  the  ether  is  not  symmetrical 
with  respect  to  a  point,  as  in  an  isotropic  medium  ;  nor  is  it  symmetrical 
with  respect  to  a  line,  as  in  uniaxial  doubly  refracting  media.  It  is, 
however, -sym metrical  to  three  planes.  If  the  elasticity  of  the  ether 
perpendicular  to  one  of  these  planes  is  the  greatest  in  the  crystal,  then 
there  must  be  within  this  plane  a  direction  which  is  parallel  to  the 
smallest  elasticity  of  ether  within  the  crystal ;  and,  moreover,  at  right 
angles  to  this  direction  of  least  elasticity,  there  must  be  a  direction  in 
the  same  plane  which  corresponds  to  an  intermediate  elasticity.  The 
distribution  of  the  elasticities  of  the  ether  within  a  crystal  not  having 
a  primary  axis  may  be  referred  to  three  directions  at  right  angles  to 
one  another,  which  are  called  the  three  axes  of  elasticity,  and  are  dis- 
tinguished as  the  axis  of  greatest  elasticity  (a),  axis  of  mean  elasticity 
(b),  and  axis  of  least  elasticity  (c).  The  form  of  the  wave-surface  of 
light  (surface  of  elasticity)  transmitted  in  crystals  which  are  without  • 
primary  axis  is  derived  in  the  following  manner: 


Till  A  XI AL  ELLIPb  u  ID. 


Let  the  length  of  the  lines  a,  b,  and  c  (Fig.  6)  be  proportional  to  the 
square  root  of  the  axes  of  greatest,  mean,  and  least  elasticity.  Suppose 
that  from  the  point  of  intersection,  0,  a  luminous  movement  advances 
in  all  directions,  and  let  us  follow  this  movement  in  the  plane  be.  In 
the  direction  ot  two  rays  will  advance  with  different  velocities,  of  which 


.  6 


one  vibrating  parallel  to  a  will  reach  a  in  a  unit  of  time,  if  (Fig.  7") 
oa  —  .Jet,  while  the  second  ray  swinging  parallel  to  b  will  reach  I.  in  a 
unit  of  time,  if  ob  —  ^b.  In  the  same  manner,  in  the  direction  ob  two 
rays  will  advance,  of  which  one  swinging  parallel  to  a  will  traverse 
the  distance  oal  =  oa  —  -£a,  in  a  unit  of  time,  while  the  second  swing- 
ing parallel  to  c  will  traverse  oc  =  -Jc.  In  every  other  direction  with- 
in the  plane  be  (Fig.  6)  two  rays  will  advance,  one  of  which  always 
swinging  parallel  to  c^  will  traverse  a  distance  0#a,  oa^  etc.,  —  oa  =  -|a  ; 
while  the  second  swinging  parallel  to  an  elasticity  lying  between  b  and 
C  (and  perpendicular  to  a)  will  traverse  a  distance  equal  to  ob^  ob^  etc., 
if  about  o  (Fig.  7)  an  ellipse  be  described  with  the  half-axes  ob  =  £b 
and  oc/=  4-C. 

Following  in  the  same  manner  the  movement  of  the  light  in  the 
plane  ab,  we  see  that  in  the  direction  #b  (Fig.  6)  two  rays  proceed,  one 
of  which  swinging  parallel  to  a  will  traverse  a  distance  oa  =  ^a  (Fig. 
8)  in  a  unit  of  time,  while  the  other  swinging  parallel  to  c  will  trav- 
f  ion  <?a  (Fig. 

{  ce  ocl9  =  |-C 

<  other  direc- 

i  a  neon  sly,  of 

aces  OC0  oc.3 ; 


36 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


while  the  other  will  advance  with  a  velocity  >b<Ct,  and  will  therefore 
pass  over  the  distances  ob»  obz,  etc.,  when  ob  —  ^b  and  oa  —  ^a  form 
the  half-diameters  of  the  ellipse. 

Finally,  for  the  movement  in  the  plane  ac  (Fig.  6)  we  find  that 
from  the  point  o  in  the  direction  6>a,  two  rajs  are  transmitted,  one  of 
which  vibrating  parallel  b  traverses  ob  (Fig.  9)  in  a  unit  of  time,  while 
the  second,  vibrating  parallel  c,  traverses  oc.  In  the  direction  0c,  the 
raj  vibrating  parallel  b  will  traverse  ob,  and  that  swinging  parallel  a 
will  traverse  oa.  For  the  movement  in  every  direction  in  the  plane  ac 
we  shall  obtain  the  corresponding  velocities  if  we  describe  about  o  a 
circle  whose  radius  ob  —  -Jb,  and  an  ellipse  with  the  half-diameters  oa 
—  |a  and  oc  =  Jc,  and  draw  radii  in  the  direction  in  question.  Since 

( 


Fig.  8 


.  9 


the  diameter  of  the  circle  equals  the  square  root  of  the  mean  elasticity,, 
and  the  diameters  of  the  ellipse  equal  the  square  root  of  the  greatest 
and  of  the  least  elasticities,  the  circle  and  ellipse  must  intersect  in  four 
points. 

The  two  rajs  traversing  the  crystal  in  the  direction  ou^  have  the 
same  velocities,  but  different  wave-surfaces  (kk  and  k.'k'\  and  there- 
fore upon  exit  from  the  crystal  will  be  differently  refracted.  On  the 
other  hand,  the  rays  along  oM  and  oT  have  the  same  wave-surface, 
when  TM is  tangent  to  both  curves;  they  will  therefore  advance  in  the 
direction  Tt,  Mm  as  a  single  wave.  The  same  is  true  of  all  the  rays 
lying  in  the  surface  of  a  cone  whose  angle  is  ToM^  for  a  plan?  through 


CONICAL  REFRACTION.  37 

TM  is  tangent  to  the  surface  of  the  ellipsoid  at  the  exit  of  all  these 
rajs,  and  its  contact  with  it  is  a  circle  whose  diameter  is  TM.  There- 
fore all  the  rays  from  o  to  the  circumference  of  this  circle  have  the 
same  wave-surface,  and  will  upon  their  exit  advance  as  a  hollow  cylin- 
der of  rays.  And  since  all  rays  traversing  the  crystal  in  the  direction 
Mm  or  Tt  possess  but  one  wave-surface,  then  on  emerging  from  the 
•crystal  they  will  not  experience  any  double  refraction.  The  direction 
normal  to  the  plane  TM  being  one  in  which  rays  traverse  the  crystal, 
and  emerge  without  being  doubly  refracted,  is  called_jvn_j2^^c.  axis. 
Therefore  the  directions  oM  and  0J/J  are  the  optic  axes,  and  such  crys- 
tals are  called  biaxial.  Moreover,  a  plane- wave  coming  from  an  iso- 
tropic  medium  in  a  direction  normal  to  the  tangential  plane  TM  must 
produce  in  the  biaxial  medium  a  cone  of  rays  which  will  emerge  again 
as  a  cylinder  of  rays ;  the  optic  axes  then  are  also  called  axes  of  the 
inner  conical  refraction . 

The  two  wave-surfaces  which  emerge  from  the  crystal  at  i^  have 
different  directions  i^v,  utv^  which  are  normal  to  the  tangent  planes 
JcJc  and  k'k' ;  they  diverge,  and,  together  with  all  those  whose  direc- 
tions are  normal  to  all  the  tangents  to  the  surface  of  the  ellipsoid  at 
the  point  ?£„  give  rise  to  a  hollow  cone  of  rays  analogous  to  conical 
internal  refraction.  This  phenomenon  is  know^n  as  conical  external 
refraction. 

This  characteristic,  as  well  as  the  fact  that  the  optic  axes  of  a  bi- 
axial medium  are  not  axes  of  symmetry  of  the  ellipsoid  of  elasticity, 
distinguish  them  essentially  from  the  optic  axis  of  uniaxial  media. 
The  movement  of  the  light  for  every  plane  which  does  not  pass 
through  two  axes  of  elasticity  of  the  triaxial  ellipsoid  can  be  followed 
out  in  the  same  manner,  and  it  will  be  seen  that  for  every  movement 
of  light  outward  from  the  centre  there  will  result  two  rays,  advancing 
with  different  velocities  and  polarized  at  right  angles  to  one  another. 
Inversely,  every  ray  entering  an  anisotropic  biaxial  medium  with  per- 
pendicular incidence  will  be  divided  into  two  rays,  which  are  polar- 
ized at  right  angles  to  one  another,  and  which,  with  the  exception  of 
those  parallel  to  an  optic  axis,  proceed  .with  different  velocities.  For 
oblique  incidence  the  two  rays  produced  by  double  refraction  will  ad- 
vance with  different  velocities  and  in  different  directions.  The  direc- 
tions of  vibration  of  the  two  parts  of  a  doubly  refracted  ray  are  the 
axes  of  the  ellipse  cut  from  the  ellipsoid  of  elasticity  by  a  central 
plane  at  right  angles  to  the  direction  of  the  incident  ray. 

Comparing  the  two  parts  of  a  doubly  refracted  ray  in  an  anistropic 
biaxial  medium  with  those  in  an  anisotropic  uniaxial  medium,  we  see 


38  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

that  none  of  the  first-mentioned  rays  have  a  constant  velocity  of  trans- 
mission, and  consequently  that  none  have  a  constant  index  of  refrac- 
tion ;  and  since  these  values  for  both  rays  change  with  the  direction,. 
they  are  both  extraordinary  rays.  Nevertheless  one  is  called  the 
ordinary  and  the  other  the  extraordinary  ray,  from  analogy  with  those 
of  uniaxial  media.  Three  principal  indices  of  refraction  are  distin- 
o-uished  in  biaxial  media:  a  is  the  index  of  refraction  of  rays  advanc- 
ing at  right  angles  to  a  and  vibrating  parallel  to  a  ;  ft  is  the  index  of 
those  advancing  perpendicular  to  b  and  vibrating  parallel  to  b  ;  and  y 
the  index  of  rays  advancing  perpendicular  to  tjt  and  vibrating  parallel  C 
to  C.  Since  the  refraction  is  inversely  proportional  to  the  square  root 
of  the  elasticity,  we  have 


•  -4 

These  indices  naturally  change  with  the  wave-length  of  the  light, 
Fig  9  shows  that  the  plane  of  the  optic  axes  in  a  biaxial  medium  must 
always  lie  in  the  plane  of  the  axes  of  greatest  and  least  elasticity  of 
this  medium,  and  that  the  angle  between  the  optic  axes  must  be 
bisected  by  these  axes.  These  axes  of  elasticity  are  therefore  generally 
called  the  bisectrices  ;  the  one  bisecting  the  acute  angle  of  the  optic 
axes  is  called  the  first  or  acute  bisectrix,  and  that  bisecting  the  obtuse 
optical  angle  is  the  second  or  obtuse  bisectrix.  The  axis  of  mean 
elasticity  stands  at  right  angles  to  the  plane  of  the  optic  axes,  and  is 
called  the  optical  normal.  The  angle  which  the  optic  axes  make  with 
one  another,  and  consequently  the  angle  each  makes  with  a  bisectrix,  is 
dependent  on  the  relative  values  of  a,  b,  and  c,  and  of  <*,  /?,  and  y.  If 
the  angle  between  one  optic  axis  and  the  axis  of  least  elasticity  is  called 
F,  then 


y* 


Now  since  the  value  of  a,  /3,  and  y  changes  with  the  wave-length  of 
the  light,  the  angle  between  the  optic  axes  and  the  bisectrix  must 


PRINCIPAL   OPTIC  SECTION,  39 

change  with  the  wave-length  also  .This  is  known  as  the  dispersion  of 
the  optic  axes,  VP<^  "^;  that  is,  the  angle  between  an  optic  axis  and 
the  bisectrix  for  red  light  is  greater  or  less  than  that  for  blue  light. 

In  every  elliptical  section  through  the  triaxial  ellipsoid  of  biaxial 
media,  which  is  not  at  right  angles  to  the  plane  of  the  optic  axes,  the 
projection  of  the  optic  axes  m  m  and  ?n1  m^  (Fig.  10)  must  be  sym- 
metrical to  the  diameter  of  the  ellipse,  which 
therefore  represents  their  bisectrix.  Now 
since  the  axes  of  the  ellipse  are  the  direc- 
tions of  vibration  of  the  two  parts  of  a 
doubly  refracted  ray  which  advances  per- 
pendicular to  the  plane  of  the  ellipse,  we 
may  lay  down  the  rule  that  the  directions 
of  vibration  of  both  rays  bisect  the  angles 
between  the  optic  axes.  Therefore  the  direction  of  vibration  of  one  part 
of  a  doubly  refracted  ray  which  strikes  perpendicular  to  the  face  of  a  crys- 
tal is  found  by  passing  a  plane  through  the  ray  (the  normal  to  the 
crystal  face)  and  the  first  bisectrix ;  the  vibrations  lie  in  this  plane  at 
right  angles  to  the  ray.  The  vibrations  of  the  second  part  of  the  ray 
must  beat  right  angles  to  those  of  the  first  part.  If  the  plane  through 
the  incident  ray  and  the  bisectrix  is  called  the  principal  optic  section, 
then  one  ray  vibrates  at  right  angles  to  this  plane  and  is  called  the 
ordinary  ray,  although  it  does  not  behave  like  the  ordinary  ray  of  a 
uniaxial  medium  ;  the  ray  vibrating  in  the  principal  section  is  called 
the  extraordinary  ray.  If  the  elliptical  section  is  at  right  angles  to  the 
plane  of  the  optic  axes,  then  these  with  the  bisectrix  and  the  principal 
optic  section  all  fall  together. 

As  two  of  the  three  axes  of  elasticity  of  a  biaxial  medium  approach 
equality,  the  angle  between  the  optic  axes  diminishes :  it  will  be  =  o, 
and  both  optic  axes  will  coincide  with  one  another  and  with  one  axis 
of  elasticity  as  soon  as  the  difference  between  the  other  two  axes  of 
elasticity  =  o.  This  theoretical  transition  of  biaxial  media  into  uniaxial 
can  take  place  by  b  equalling  c  or  by  a  equalling  b.  In  the  first  case 
there  arises  an  optically  negative  uniaxial  crystal,  and  in  the  second  a 
positive  one ;  therefore  in  optically  biaxial  crystals  those  are  consid- 
ered as  negative  in  which  the  axis  of  greatest  elasticity  is  the  acute  bi- 
sectrix, and  those  as  positive  in  which  the  axis  of  least  elasticity  is  the 
acute  bisectrix. 


40 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


Optical  Characteristics  of  the  Three  Crystal  Systems  without  a 

Primary  Axis. 

Just  as  the  crystal  systems  without  a  primary  axis  are  distinguished 
from  isotropic  media  by  their  double  refraction,  and  from  crystals 
with  a  primary  axis  by  their  having  two  optic  axes,  so  they  are  dis- 
tinguished from  one  another  by  the  orientation  of  their  ellipsoids  of 
elasticity  with  respect  to  their  crystallographic  constants,  and  the  con- 
sequent dispersion  of  their  axes. 

In  the  orthorhombic  system  the  three  axes  of  elasticity  (a,  b,  c) 
coincide  with  the  crystallographic  axes  of  symmetry  (a,  &,  c)  because 
of  the  correspondence  between  the  morphological  and  physical  sym- 
metry ;  any  one  of  the  first  coinciding  with  any  one  of  the  second, 
without  there  being  any  connection  whatever  between  the  relative 
lengths  of  either  group  of  axes.  Such  a  connection  is  excluded  by  the 
fact  that  the  choice  of  the  vertical  axis  and  of  the  fundamental  form  is 
arbitrary.  But  since  every  axis  of  elasticity  coincides  with  a  crystal- 
lographic axis  of  symmetry,  a  proper  dispersion  of  the  axes  of  elas- 
ticity (bisectrices)  is  rendered  impossible.  However,  this  does  not 
prevent  in  one  and  the  same  crystal,  as  for  instance  in  brookite,  the 
bisectrices  of  the  optic  axes  for  light  of  different  wave-lengths  from 
coinciding  with  different  crystallographic  axes 
of  symmetry.  The  plane  of  the  optic  axes  always 
lies  in  one  of  the  pinacoids,  and  light  of  different 
wave-lengths  is  dispersed  symmetrically  with 
respect  to  both  bisectrices.  Fig.  11  is  the  optical 
scheme  for  an  orthorhombic  crystal  ( ex  P,  oP) 
with  optically  negative  character,  whose  axes  lie 
in  the  macrodiagonal  (principal)  section  with  the 
vertical  axis  as  the  first  or  acute  bisectrix.  The 
dispersion  is  p  >  v . 

In  the  monodinic  system  only  one  of  the 
so-called  crystal  axes,  the  orthodiagonal  1>,  is  an 
actual  axis,  that  is,  the  normal  to  a  plane  of  sym- 
metry. This  must  therefore  always  coincide 
with  one  of  the  axes  of  elasticity,  which  naturally  suffers  no  dispersion, 
and  is  an  axis  of  elasticity  for  light  of  all  wave-lengths.  The  two  other 
axes  must  lie  in  the  clinopinacoid,  because  they  are  at  right  angles  to  £>, 
and  since  they  correspond  to  no  morphological  axes  of  symmetry, 
they  must  generally  suffer  a  small  dispersion,  so  that  they  have  differ- 
ent positions  for  rays  of  different  colors.  The  plane  of  the  optic  axes 


D1SPKRS10N  OF  THE  OPTIC  AXES.  41 

must  either  lie  in  the  plane  of  symmetry  or  at  right  angles  to  it, 
for  in  every  case  &  is  an  axis  of  elasticity.  According  to  the  optical 
value  of  5,  two  groups  of  crystals  are  distinguished. 

(1)  &  =  b ;  the  orthodiagonal  is  the  axis 
of  mean  elasticity ;  it  is  so  for  all  colors. 
The  axes  of  greatest  and  least  elasticity  (bi- 
sectrices) for  different  colors  lie  dispersed 
in  the  plane  of  symmetry,  in   which    also 
the  optic  axes  for  different  colors  are  dis- 
persed symmetrically  with  respect  to  their 
corresponding  bisectrices.    There  is  no  com- 
mon  bisectrix  for  all  wave-lengths.     This 
kind  of  dispersion  is  called  inclined  disper- 
sion.    Fig.  12  presents  the  scheme  of   an 
optically  positive  crystal  with  inclined  dis- 
persion, in  which  rp  (the  positive   bisectrix 
for  red  rays)  has  a  greater  inclination  with 
respect  to  the  vertical  axis  than  cv  (the  pos- 
itive bisectrix  for  blue  rays).     The  inclined 
dispersion  presents  the  most  widely  spread 
form  of  optical   orientation  of   monoclinic 
crystals ;   such   crystals  are  said  to  have  a 
symmetrical  position .-of  the  axes. 

(2)  1)  —  a  or  c;  the  orthodiagonal  is  the  axis  of  greatest  or  least  elas- 
ticity, and  is  therefore  one  of  the  two  bisectrices.     Then  the  plane  of 
the  optic  axes  must  lie  at  right  angles  to  the  plane  of  symmetry ;  these 
crystals  have  normal  symmetrical  position   of   the  axes.     They  are 
divided  into  two  groups,  according  as  the  orthodiagonal  is  the  obtuse 
or  the  acute  bisectrix.^  If  the  orthodiagonal  is  the  obtuse  bisectrix, 
there  cannot  be  any  dispersion  of  this  bisectrix  and  the  planes  of  the 
optic  axes,  for  all  colors  must  pass  through  the  axis  1).     The  dispersion 
is  confined  to  the  acute    bisectrix   and   the   axis  of  mean  elasticity. 
Looked  at  in  a  direction  at  right  angles  to  I,  the  planes  of  the'optic 
axes  for  different  wave-lengths  lie  horizontally  over  one  another,  for 
which  reason  this  form  of  dispersion  is  called  the  horizontal  dispersion. 
The  scheme  for  this  is  shown  in  Fig.  13. 

If  the  orthodiagonal  is  the  acute  bisectrix,  only  those  axes  of  elas- 
ticity, the  second  bisectrix  and  the  normal,  which  lie  in  the  plane  of 
symmetry  can  be  dispersed,  and  the  planes  o*f  the  optic  axes  perpendicu- 
lar to  the  symmetry  plane  must  cross  one  another.  Fig.  13  shows  this 
kind  of  dispersion,  which  is  crossed  dispersion,  if  looked  at  in  the 


42 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


direction  of  b.  Horizontal  and  crossed  dispersion  constantly  occur  to- 
gether, and  it  depends  only  on  the  size  of  the  optic  angle  whether  one 
or  the  other  kind  of  dispersion  is  ascribed  to  a  substance. 


13 


Since  in  the  tridinic  or  asymmetric  system  the  so-called  crystal 
axes  are  arbitrarily  chosen  co-ordinates,  there  can  no  longer  exist 
between  the  axes  of  elasticity  and  the  crystal  axes  any  definite  re- 
lationship. In  general  these  directions  do  not  fall  together.  For 
this  reason  there  occurs  a  dispersion  of  all  the  axes  of  elasticity,  and 
the  ellipsoids  of  elasticity  for  light  of  different  wave-lengths  have  no- 
axes  in  common.  This  gives  rise  to  the  simultaneous  occurrence  of 
several  axial  dispersions. 

Influence  of  Temperature  and  Pressure  on  Double  Refraction. 

As  in  isotropic  media  the  index  of  refraction  changes  with  the 
temperature  and  pressure,  so  in  anisotropic  substances  there  is  a 
dependence  of  the  optical  constants  on  pressure  and  heat,  which  shows 
itself  partly  in  a  change  in  the  absolute  size  of  the  index  of  refraction 
of  a  particular  axis  of  elasticity,  and  consequently  in  the  relative  size 
of  the  two  or  three  principal  indices  of  refraction  peculiar  to  an  aniso- 
tropic substance,  and  partly  in  a  change  of  position  of  the  optical 
constants.  So  long  as  the  changes  of  temperature  in  all  parts  of  an 
anisotropic  medium  are  the  same,  and  its  molecular  construction  and 
chemical  composition  remain  the  same,  all  the  variations  of  the  optical 
ellipsoid  of  elasticity  occur  in  such  a  manner  that  this  possesses  at  all 
temperatures  the  degree  of  symmetry  corresponding  to  the  crystal 
form  of  the  medium.  Consequently  the  ellipsoid  of  rotation  of  a 
uniaxial  substance  remains  an  ellipsoid  of  rotation  for  all  temperature^ 


INFLUENCE  OF  TEMPERATURE  AND  PRESSURE.  43 

and  never  passes  into  a  sphere  for  all  kinds  of  light  at  any  one  time, 
nor  hecomes  the  triaxial  ellipsoid  of  biaxial  media. 

In  the  same  way  the  triaxial  ellipsoid  of  a  biaxial  body  remains 
such  for  all  temperatures,  or  may  become  a  rotation  ellipsoid  for  each 
kind  of  light  only  at  different  temperatures,  never  for  all  kinds  of  light 
at  one  and  the  same  temperature.  Moreover,  the  axes  of  this  triaxial 
ellipsoid  are  constant  in  their  position  so  long  as  they  coincide  with 
crystallographic  axes  of  symmetry.  Therefore,  in  an  orthorhombic 
body,  the  optical  variations  due  to  heating  must  be  confined  to  the 
relative  value  of  the  three  axes  of  elasticity,  and  consequently  to  the 
size  of  the  ang^  and  position  of  the  plane  of  the  optic  axes. 

With  monoclinic  crystals  because  of  the  variations  in  the  relative 
value  of  the  three  principal  coefficients  of  elasticity  which  are  often 
considerable,  and  the  consequent  angle  of  the  optic  axes,  there  occurs 
not  only  a  transition  of  the  optic  axial  plane  from  normal  symmetrical 
into  symmetrical  position  or  the  reverse,  but  the  triaxial  ellipsoid  of 
elasticity  may  be  revolved  about  the  symmetry  axis  common  to  itself 
and  the  crystal,  and  thus  a  change  in  the  position  of  two  axes  of  elas- 
ticity take  place. 

In  the  triclinic  system  the  only  limitation  to  the  optical  variations 
produced  by  heating  is,  theoretically,  that  for  every  temperature  the 
elasticity  of  the  ether  must  be  expressed  by  a  triaxial  ellipsoid.  The 
size  and  position  of  the  three  axes  is,  theoretically,  wholly  variable.  In 
actual  fact,  for  the  few  triclinic  substances  which  have  been  investi- 
gated in  this  direction  a  great  constancy  in  the  optical  relations  for 
variations  of  temperature  has  been  found. 

A  uniform  pressure  acting  on  all  sides  of  a  body  must  produce 
optical  effects  which  would  be  subjected  to  the  same  regular  varia- 
tions. 

In  a  great  number  of  the  cases  investigated  the  crystal  system  of 
the  substance  and  the  uniformity  of  the  variations  in  the  optical  ellip- 
soid of  elasticity  produced  by  heating  remain  the  same. 

But  there  is  a  considerable  number  of  so-called  "  mimetic  crystals" 
in  which  the  outward  crystal  form  appears  to  stand  in  more  or  less 
striking  contradiction  to  their  physical  and  especially  to  their  optical 
behavior.  These  substances  are  characterized  almost  without  excep- 
tion by  a  very  complicated  twinning  structure.  Because  of  this 
apparent  contradiction  they  are  also  called  optically  anomalous  crystals. 
To  these  belong  many  garnets,  alums,  senarmontite,  boracite,  perof- 
skite,  analcite,  leucite,  tridymite,  etc. 

According  to  whether  the  outward  form  or  the  optical  behavior  of 


44  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

these  substances  is  considered  to  have  the  greater  weight  in  deter m in 
ing  their  crystal  system,  the  apparent  contradiction  is  explained  either 
as  the  result  of  strains  which  have  disturbed  the  normal  physical  con- 
ditions belonging  to  the  present  crystal  form,  or  by  supposing  that 
many  small  individuals  of  a  lower  crystallonomic  symmetry  have  been 
combined  by  twinning  to  a  compound  individual  of  apparently  greater 
crystallonomic  symmetry.  The  latter  view  is  specially  strengthened 
by  the  fact  that  without  exception  the  physical  symmetry  of  such 
mimetic  structure  is  of  a  lower  order  than  the  crystallonomic,  while 
there  appears  to  be  no  grounds  a  priori  why  strains  of  themselves 
should  not  convert  a  less  symmetrical  physical  condition  into  a  more 
.symmetrical  one. 

The  numerous  studies  of  many  investigators  on  these  pseudosyrn- 
rnetrical  or  mimetic  forms  have  shown  that  a  great  number  of  these 
apparent  anomalies  may  be  made  to  disappear  upon  heating.  This  is 
explained  by  the  fact  that  such  mimetic  substances  are  dimorphous, 
and  assumed  a  form  through  the  physical  conditions  accompanying 
their  genesis  which  is  not  the  position  of  equilibrium  of  their  mole- 
cular structure,  conformable  with  the  subsequent  physical  conditions 
in  which  they  now  exist.  There  arises  therefore,  with  the  changed  con- 
ditions of  existence,  a  molecular  alteration  within  the  outward  crystal 
form  originally  assumed,  and  which  is  more  or  less  permanent,  by 
which  the  crystal  endeavors  to  approach  as  near  as  possible  to  a  co  ndi 
tion  of  equilibrium  corresponding  to  the  altered  conditions.  Whether 
this  is  actually  attained, — that  is,  whether  the  symmetry  of  a  mimetic 
body  indicated  by  optical  investigation  is  actually  the  one  which  cor- 
responds to  present  existing  conditions  of  pressure  and  temperature, 
or  whether  it  is  only  occasioned  by  certain  strains  which  may  arise 
through  the  exertions  of  a  new  molecular  state  of  equilibrium  within 
?.n  unyielding,  rigid,  outer  form, — is  not  always  easy  to  determine  in 
any  given  case. 

For  example,  if  we  see  plates  of  tridymite,  which  from  their  gonio- 
metric  behavior  are  hexagonal,  resolved  optically  into  parts  which  show 
the  phenomena  of  triclinic  penetration  twins,  and  if  we  find  that  at 
sufficiently  elevated  temperatures  these  plates  show  the  normal  optical 
phenomena  of  uniaxial  crystals  flattened  parallel  to  the  base,  the  con- 
clusion is  certainly  correct  that  we  have  in  tridymite  a  holohedral 
hexagonal  form  of  silica,  and  that  this  form  under  certain  conditions 
of  high  temperature  presents  the  normal  form  of  silica.  But  it 
would  be  incorrect  to  conclude  that  there  is  a  triclinic  form  of  silica 
capable  of  being  formed  under  ordinary  temperature  and  simple 


LATERAL  PRESSURE  AND  IRREGULAR  HEATING.  45 


atmospheric  pressure.  Much  rather  may  the  apparent  twinning  as- 
well  as  the  apparent  triclinic  optical  behavior  be  explained  by  an 
abnormal  condition  of  strain,  which  arises  in  the  tridymite  plate  from 
the  fact  that  a  molecular  alteration,  possibly  to  the  quartz  form  or  to 
some  unknown  modification,  is  attempted,  but  is  not  attained  because 
the  rigidity  of  the  outer  form  prevents  it.  Such  a  strain  would  act  in 
the  same  way  as  an  irregular  lateral  pressure  or  a  many-sided  unequal 
pressure. 

Lateral  pressure  and  irregular  heating  change  the  optical  elasticity 
in  an  abnormal  manner,  and  produce  a  contradiction  between  the  crys- 
tallographic  form  and  the  optical  behavior.     Isotropic,  that  is.  amor 
phous  and  isometric,  bodies  become  anisotropic  through  lateral  pres- 
sure or  unequal  heating,  and  there  occurs  a  distribution  of  the  optical 
elasticity,  which  expresses  itself  sometimes  in  an  ellipsoid  of  rotation, 
sometimes*  in    a   triaxial   ellipsoid.      They   thus   become  uniaxial  or 
biaxial ;  arid  Brewster  has  shown  that  the  occurrence  of  one  or  the 
other  alteration  is  determined  essentially  by  the  form  of  the  isotropic 
body.      In  the  same  way  he   found    that  optically  uniaxial  crystals 
which   are   compressed   at   right   angles   to   their  optic  axis  become 
biaxial ;  and  Moiguo  and  Pfaff  showed  that  with  positive  crystals  the 
plane  of  the  optic  axes  stands  parallel  to  the  direction  of  pressure, 
and  with  negative  crystals  at  right  angles  to  it.     This  behavior  is  ex- 
plained by  the  fact  that  pressure  increases  the  elasticity,  and  the  plane 
of  the  optic  axes  must  lie  in  the  plane  of  the  axes  of  greatest  and 
least  elasticity.     Since  in  positive  crystals  c  =  c,  then  the  original  elas- 
ticity which  is  the  greatest  in  all  directions  perpendicular  to  c  becomes 
still  greater  in  the  direction  of  pressure,  and  at  right  angles  to  this  it 
remains  unaltered  ;  the  plane  of  the  optic  axes  therefore  passes  through 
the  primary  axis  and  the  direction  of  pressure.     It  is  the  reverse  when 
c  =  a      H.  Bucking  found  that  a  small  pressure  is  sufficient  to  bring 
about  a  biaxial  condition,  but  that  the  pressure  must  be  considerably 
greater  to  increase  the  axial  angle   afterwards.     The  differences   of 
elasticity  due  to  pressure,   therefore,    are   not    directly  proportional 
to  the  pressure.     He  also  investigated  the  effect  of  pressure  on  biaxial 
sanidine,  and  found  that  a  pressure  parallel  to  the  axis  of  mean  elas- 
ticity diminishes  the  angle  of  the  optic  axes  when  they  lie  at  right 
angles  to  the  clinopinacoid,  and  increases  it  when  they  lie  in  the  clino- 
pinacoid  ;  that  is,  it  acts  like  a  uniform  hearing  (increase  of  tempera- 
ture).    "W.  Klein  showed  that  a  lateral  heating  perpendicular  to  the 
primary  axis  converts  a  uniaxial  crystal  into  a  biaxial  one,  and  in  such 
a  manner  that  the  elasticity  becomes  smaller  in  the  direction  of  the 


46  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS.   . 

application  of  the  heat.  Upon  the  lateral  heating  of  plates  of  biaxial 
crystals  cut  at  right  angles  to  the  bisectrix  a  deformation  of  the  ellip- 
soid of  elasticity  takes  place,  until  the  heating  becomes  uniform  ; 
when  the  alterations  are  those  shown  in  uniformly  heated  plates. 

c.  Investigation  of  Minerals  in  Parallel  Polarized  Light. 

Ordinary  light  in  its  passage  through  doubly  refracting  media  in 
any  direction  except  that  of  an  optic  axis  is  always  separated  into  two 
rays,  which  are  polarized  at  right  angles  to  one  another,  and  are  gener- 
ally transmitted  with  different  velocities  and  in  different  directions. 
It  differs  essentially  from  a  ray  of  polarized  light  in  that  the  latter  is 
not  separated  into  two,  if  its  plane  of  vibration  is  parallel  to  or  per- 
pendicular to  the  principal  optical  plane  of  the  doubly  refracting  me- 
dium through  which  it  passes.  In  such  cases  the  polarized  ray  only 
suffers  a  change  of  velocity.  But  if  the  plane  of  vibration  of  the  po- 
larized light  makes  any  other  angle  than  0°  or  90°  with  the  principal 
section  of  the  anistropic  medium,  it  is  separated  into  two  rays  perpen- 
dicular to  one  another,  which  are  generally  transmitted  with  different 
velocities  in  different  directions,  just  as  in  the  case  of  ordinary  light. 
The  ray  of  polarized  light  becomes  depolarized  or  rather  repolarized. 

In  consequence  of  the  fact  that  polarized  light  is  not  separated  into 
parts  when  its  plane  of  vibration  is  parallel  or  perpendicular  to  the 
principal  plane  of  the  medium  traversed,  and  because  of  the  interfer- 
ence of  the  separated  rays  in  all  other  positions  when  the  vibrations 
are  reduced  to  one  plane,  doubly  refracting  media  exhibit  certain  dif- 
ferences from  singly  refracting  ones,  and  give  rise  to  interference  phe- 
nomena when  investigated  in  polarized  light,  which  lead  not  only  to 
the  distinction  of  isotropic  and  anistropic  media,  but  also  to  the  deter 
mination  of  the  position  of  the  axes  of  elasticity  and  of  the  optic  axes. 
Now  since  the  position  of  the  axes  of  elasticity,  as  already  pointed  out, 
stands  in  the  closest  relation  to  the  crystal  structure  of  the  media,  so 
the  optical  investigation  makes  possible  a  determination  of  the  crystal 
system  with  the  same  or  even  greater  sharpness  than  the  goniometric 
investigation  does.  For  establishing  the  position  of  the  axes  of  elas- 
ticity and  the  distinguishing  of  isotropic  and  anisotropic  media,  the 
investigation  in  parallel  polarized  light  is  to  be  preferred ;  convergent 
polarized  light  is  used  for  determining  the  optic  axes  and  their  incli- 
nation. «  / 

Polarizing  Instruments. — Every  instrument  by  which  refracting 
media  may  be  investigated  in  polarized  light  is  called  a  polarizing  in* 


TOURMALINS  TO  NO  8. 

SSSument :  it  always  consists  of  two  parts.  The  first  part  transfon 
ordinary  light  into  polarized  light,  and  is  called  tire  polarizer;  t 
second  part  tests  or  analyzes  the  polarized  light  either  by  itself  or  afi 
its  passage  through  the  medium  under  investigation  :  it  is  called  t 
analyzer.  To  transform  ordinary  light  into  polarized  light,  it  m 
either  be  reflected  at  the  Brewster  angle  from  a  non-metallic  mirror, 
be  allowed  to  pass  through  a  doubly  refracting  medium  in  any  dir 
tion  but  that  of  an  optic  axis.  One  of  the  two  polarized  rays  tl 
produced  must  then  be  eliminated. 

The  simplest  polarizing  instrument,  the  tourmaline  tongs,  consi 
of  two  brown  or  dark-green  tourmaline  plates  cut  parallel  to  the  pr 
cipal  section,  and  set  in  frames  which  may  be  rotated  in  the  end  rii 
of  elastic  wires  bent  into  the  shape  of  shears.     The  mineral  under 
vestigation  is  held  between  the  tourmaline  plates.     If  a  ray  of  ordim 
light  falls  on  the  first  tourmaline  plate  it  will  be  divided  into  two  ra 
according  to  the  laws  governing  the  movement  of  light  in  uniaxial  \ 
dia  :  these  rays  will  advance  parallel  to  each  other  for  perpendicular 
cidence,  but  with  different  velocities,  one  vibrating  parallel  to  the  - 
tic  axis  (E\  the  other  vibrating  at  right  angles  to  it  (0).     Now  sii 
tourmaline  possesses  the  property  of  extinguishing  the  vibrations 
right  angles  to  its  optic  axis, — that  is,  of  absorbing  the  ordinary  r 
when  the  plate  is  sufficiently  thick, — there  emerges  from  it  only  a 
vibrating  parallel  to  c.     The  tourmaline  plate  is  thus  a  polarizer; 
dinary  light  upon  entering  it  is  transformed  through  double  refract 
and  absorption  into  polarized  light,  whose  plane  of  vibration  is  knm 
If  the  second  tourmaline  plate,  which  is  to  serve  as  an  analyzer,  i^ 
placed  that  its  optic  axis  is  parallel  to  that  of  the  polarizer,  then 
extraordinary  ray  which  comes  from   the  polarizer,  and  whose  pi 
of  vibration  lies  parallel  to  the  principal  section  of  the  analyzer,  A 
experience  no  separation,  but  will  also  pass  through  the  second  toun 
line  plate  as  an  extraordinary  fay  with  unchanged  direction  of  vil 
tion.      If  one  looks  through  both  plates  in  this  position   (with 
principal  sections  parallel)  there  is  a  uniform  green  or  brown  field 
view,  as  though  there  were  but  a  single  tourmaline  plate. 

If  the  analyzer  is  rotated  until  its  principal  section  stands  at  ri 
angles  to  that  of  the  polarizer,  the  extraordinary  ray,  which  emer 
from  the  latter,  will  still  be  undivided,  as  its  plane  of  vibration  r 
stands  at  right  angles  to  the  principal  section  of  the  analyzer;  it  ^ 
enter  the  latter  with  unaltered  direction  of  vibration.  The  ray  t 
traverses  the  analyzer  with  vibrations  at  right  angles  to  the  optio  a 
that  is,  as  an  ordinary  ray.  It  will  therefore  be  absorbed,  and  will 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


pass  through  it.  If  one  looks  through  the  tourmaline  plates  in  this- 
position  (with  the  principal  sections  crossed  at  right  angles)  the  field 
of  view  will  be  dark.  In  every  other  position  of  the  polarizer  with 
respect  to  the  analyzer,  the  ray  emerging  from  the  former,  its  plane  of 
vibration  being  no  longer  parallel  or  perpendicular  to  the  principal 
section  of  the  analyzer,  will  be  separated  in  the  same  manner  as  a  ray 
of  ordinary  light — that  is,  into  an  ordinary  ray,  which  vibrates  at  right 
angles  to  the  optic  axis  of  the  analyzer  and  is  absorbed ;  and  an  extra- 
ordinary one,  which  vibrates  parallel  to  the  optic  axis  and  passes 
through.  The  component  of  the  light  coming  from  the  polarizer 
which  forms  the  extraordinary  ray,  that  is,  the  intensity  of  the  light 
emerging  from  the  analyzer,  must  naturally  be  dependent  on  the  inten- 
sity of  the  incident  ray  and  the  inclination  of  the  principal  optic  sec- 
tions of  the  polarizer  and  analyzer  to  one  another.  It  is  proportional 

to  the  cosine  of  this  inclination.  Let 
ab  (Fig.  14)  be  the  principal  section  of 
the  polarizer,  ^that  of  the  analyzer,  x 
the  angle  included  between  them.  Let 
mg  =  1  represent  the  amplitude  of  vi- 
bration of  the  ray  emerging  from  the 
polarizer;  then,  if  gh  is  perpendicular  to 
ef,  this  ray  will  be  separated  in  the  ana- 
lyzer into  an  ordinary  ray  vibrating  at 
right  angles  to  ef  with  the  amplitude 
hg,  which  is  absorbed,  and  into  an  ex- 
traordinary ray  vibrating  parallel  to  ef 
with  the  amplitude  hm  —  /,,  which 
passes  through  the  analyzer.  Then 

km  =  mg  .  cos  x ; 
7^     =  / .  cos  x. 

0°,  that  is,  if  the  principal  sections  of  the  polarizer  and  analyzer 
are  parallel,  /,  =  1 ;  f or  x  =  90°,  that  is,  when  the  principal  sections 
are  crossed  at  right  angles,  /,  =  0.  The  intensity  of  light  is  propor- 
tional to  the  square  of  the  amplitude  of  vibration. 

The  deep  color  of  tourmaline  renders  it  unfit  for  use  in  micro- 
scopical investigations,  and  it  is  generally  replaced  by  the  nicol  prism. 
Such  a  nicol  prism  is  made  from  a  natural  cleavage  piece  of  calcite 
tvhich  is  three  times  as  long  as  thick.     The  upper  and  lower  faces  of 
"•liedron,  which  make  angles  of  Tl°  and  109°  with  the  edges 
.oipal  section,  are  replaced  by  others  whose  inclination 


14 


NICOL  PRISM. 


49 


to  these  edges  is  68°  and  112°;  the  rhombohedron  is  then  sawn  across 
at  right  angles  to  the  principal  section  and  to  these  newly  cut  faces, 
and  the  faces  of  the  section  after  being  thoroughly  polished  are 
cemented  together  in  their  original  position  by  Canada  balsam.  The 
cross-section  of  such  a  nicol  prism  in  the  principal  section  is  shown  in 
Fig.  15.  It  is  blackened  on  the  outside,  and  fastened 
with  a  cork  in  a  metal  tube. 

If  now  a  ray  of  light,  win,  parallel  to  the  long  edge 
of  the  prism  falls  upon  the  end  face  of  the  same,  then  it 
will  be  separated  within  the  prism  into  an  ordinary  ray, 
no,  with  an  index  of  refraction  of  1.658,  and  into  an  ex- 
traordinary ray  with  a  considerably  smaller  index  of  re- 
fraction. The  index  of  refraction  of  the  Canada  balsam 
is  1.536  ;  from  which  the  critical  angle  for  the  transition 
of  the  ordinary  ray  is  found  to  be  67°  53'.  Now  since 
the  angle  of  incidence  of  the  light  is  90°  —  68°  =  22°, 
then  the  angle  of  refraction  in  calcite  is  13°  4'  and  the 
angle  of  incidence  on  the  layer  of  balsam  is  76°  56',  and 
the  ordinary  ray  must  therefore  experience  a  total  reflec- 
tion in  the  direction  oor  The  extraordinary  ray  traverses 
the  layer  of  balsam  and  the  second  half  of  the  prism,  and  emerges 
at  q  in  the  direction  qe  parallel  to  mn.  Its  plane  of  vibration  lies 
parallel  to  the  short  or  inclined  diagonal  of  the  end  faces  of  the  , 
nicol  prism,  which  has  a  rhombic  form.  Two  nicol  prisms  act  in 
exactly  the  same  manner  as  two  tourmaline  plates ;  the  extraordi- 
nary ray  which  emerges  from  the  first  nicol  will  experience  no 
separation  in  the  second  prism,  which  serves  as  an  analyzer,  if 
its  principal  section  is  parallel  or  at  right  angles  to  that  of  the 
first.  In  a  parallel  position  the  ray  traverses  the  analyzer  as  an  extra- 
ordinary ray,  and  suffers  no  total  reflection  from  the  layer  of  balsam. 
The  field  of  view  is  completely  clear.  In  a  crossed  position  the  extra- 
ordinary ray  coming  from  the  polarizer  is  converted  into  an  ordinary 
ray  in  the  analyzer,  and  experiences  total  reflection  from  the  layer  of 
balsam.  The  field  of  view  is  dark.  For  an  inclined  position  of  the. 
principal  sections  of  both  nicols  to  one  another  we  must  have  /,  = 
1.  cos  a?,  where  x  is  the  inclination  of  the  principal  sections  to  one 
another,  and  the  illumination  of  the  field  of  view  is  I*.  cos2  x. 

The  original  construction  of  the  nicols  has  been  modified  in  various 
ways,  resulting  in  the  shortening  of  the  prism,  the  strengthening  of  the^  v  , 
transmitted  light,  together  with  the  more  complete  polarization  even 
of  the  inclined  incident  rays. 
4 


50  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

In  order  to  apply  the  microscope  to  investigations  in  polarized 
light,  a  nicol  prism  is  inserted  in  the  path  of  the  light  between  the 
mirror  and  the  object  to  serve  as  a  polarizer,  and  a  second  as  analyzer 
is  placed  between  the  object  and  the  eye  of  the  observer,  either  within 
the  tube  or  above  the  ocular  lens.  A  microscope  thus  furnished  with 
nicols  is  called  a  polarizing  microscope.  The  insertion  of  the  nicols, 
however,  only  accomplishes  the  de^jred  results  when  the  instrument 
satisfies  the  following  conditions :  (l)^The  object  under  investigation 
must  be  capable  of  rotation  in  its  ownmlane  about  the  optical  axis  of 
the  instrument  while  the  nicols  reinain|2rossed ;  (2)  The  angle  between 
any  two  positions  of  the  object  must  be  measurable  with  requisite 
accuracy ;  (3)  The  principal  sections  of  the  nicols  must  have  a  known 
position,  which  may  be  restored  after  being  displaced. 

The  general  construction  of  a  polarizing  microscope  may  be  learned 
from  the  description  of  that  manufactured  by  Cachet  et  fils  of  Paris : 
it  differs  from  others  in  having  the  ocular  wholly  independent  of  the 
objective  (Fig.  16).  The  tube  is  cut  across,  and  the  lower  part  B,  bear- 
ing the  objective,  is  united  to  <  the  rotating  stage  of  the  microscope. 
Thus  the  objective  follows  the  movement  of  the  stage  during  its  rota- 
tion, and  consequently  every  point  of  the  thin  section  which  has  been 
brought  to  the  intersection  point  of  the  cross  wires  remains  in  the  cen- 
tre of  the  field  of  view  for  every  position  of  the  stage  ;  it  cannot  rotate 
other  than  concentrically.  The  rough  adjustment  of  the  objective  is 
effected  by  the  rack-and-pinion  movement  above  Z,  the  fine  adjust- 
ment by  the  micrometer-screw  Z.  The  head  of  the  latter  is  divided 
into  100  parts,  its  position  is  read  with  a  vernier  to  the  tenth  of  a  part ; 
the  height  of  the  thread  (the  pitch)  of  the  screw  is  0.25  mm. 

The  objective  is  not  screwed  on,  but  is  held  in  place  by  a  spring  in 
a  very  convenient  and  solid  manner.  The  upper  part  of  the  tube  is 
held  firmly  by  the  outer  (bent)  metal  column,  and  is  also  raised  and 
lowered  by  means  of  a  rack-and-pinion  movement.  At  the  upper  end 
is  an  opening  through  which  the  cross  wires  in  the  ocular  may  be  il- 
luminated by  means  of  a  mirror  M ;  this  is  sometimes  desirable  when 
the  nicols  are  crossed,  and  the  field  of  view  is  very  dark.  At  the 
lower  end  of  the  ocular  tube,  in  front,  is  a  second  larger  opening  into 
which  the  analyzer  A  can  be  moved.  When  this  is  not  in  use  the  open- 
ing may  be  closed  by  means  of  a  sleeve.  At  the  extreme  end  of  the 
ocular  tube  at  II  is  a  slit  in  which  may  be  inserted  a  quartz  plate  for 
observation  with  the  sensitive  tint,  or  Bertrand's  lenses  for  magnify- 
ing interference  figures. 

The  stage  of  the  microscope  consists  of  a  circular  plate,  which  can 


POLARIZING  MICROSCOPE. 


51 


be  rotated  either  by  the  hand  or  by  the  screw  E,  which  works  when  it 
is  pushed  forward,  and  can  be  thrown  out  of  gear  by  being  pulled 
back.  The  rotating  plate  carries  a  vernier  jP,  which  moves  upon  the 


Trig.  16 


circular  scale  on  the  rim  of  the  lower  stationary  plate  of  the  stage,  and 
reads  to  the  tenth  of  a  degree.  The  object  does  not  lie  directly  on  the 
rotating  plate,  but  on  the  mechanical  stage  ./>,  which  is  moved  by  means 
of  the  screws  H  and  R'  working  at  right  angles  to  one  another.  Thus 


52  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

the  thin  section  is  not  moved  by  the  hand,  but  mechanically  ;  and  every 
point  of  it  can  be  brought  into  the  centre  of  the  field  without  any  spot 
of  the  thin  section  escaping  observation.  The  movement  of  the  object 
by  the  screws  R  R'  can  be  read  off  on  linear  scales,  on  which  the  me- 
chanical stage  I)  glides.  The  thin  section  rests  against  the  small  ledge 
F,  and  is  held  by  two  weak  springs.  Beneath  the  object-table  is  the 
tube  T,  which  can  be  raised  and  lowered  by  means  of  the  rack-and- 
pinion  c,  and  when  lowered  may  be  pushed  aside  to  Tf  for  the  purpose 
of  changing  the  apparatus  for  illumination.  In  the  tube  are  placed  the 
polarizer,  and  the  condensing  lenses  used  for  investigations  with  differ- 
ent magnifying  powers  and  in  parallel  or  convergent  light. 

Isotropic  Mineral  Plates  in  Parallel  Polarised  Light. 

If  a  thin  plate  of  an  isotropic  mineral  (amorphous  or  crystallizing 
in  the  isometric  system)  be  placed  in  the  path  of  a  polarized  ray  be- 
tween the  polarizer  and  analyzer  the  ray  of  light  will  experience  no 
alteration  of  its  plane  of  vibration,  no  matter  in  what  direction  the 
plate  was  cat  from  the  mineral,  nor  in  what  position  it  lies  between 
the  polarizer  and  analyzer.  Since  the  elasticity  of  the  ether  is  the 
same  in  all  directions  through  such  a  mineral,  its  rotation  about  any 
axis  whatever  will  effect  no  change  of  the  plane  of  vibration  of  the 
polarized  light.  If  the  mineral  is  also  colorless,  it  will  not  influence  the 
color  or  the  brightness  of  the  field  of  view,  except  for  the  small  absorp- 
tion which  a  ray  of  light  experiences  in  passing  through  any  medium  5 
if  it  is  colored,  the  field  of  view  will  show  a  color  somewhat  different 
from  that  of  the  mineral.  But  this  color  does  not  change  in  any  man- 
ner with  the  position  of  the  plate.  Moreover,  the  direction  of  the  ray 
will  not  be  changed  if  the  plate  has  parallel  faces,  and  is  set  at  right 
angles  to  the  direction  of  the  ray  ;  if  the  latter  is  not  the  case,  the  ray 
within  the  plate  will  be  deflected  from  its  course,  but  on  emerging 
from  the  plate  will  advance  parallel  to  its  direction  at  incidence.  If 
the  faces  of  the  plate  are  inclined  to  one  another,  the  direction  of  the 
ray  after  leaving  the  plate  will  differ  from  that  at  incidence  in  propor- 
tion to  the  inclination  of  the  two  faces.  Assuming  that  the  principal 
sections  of  the  analyzer  and  polarizer  are  in  crossed  position,  then 
the  consequent  darkness  of  the  field  of  view  will  not  be  disturbed  by 
the  insertion  of  an  isotropic  plate.  This  property  of  remaining  dark 
in  every  position  between  crossed  nicols,  and  for  a  rotation  of  360°  in 
its  own  plane,  is  the  most  important  characteristic  of  an  isotropic 
plate  in  contra- distinction  to  an  anisotropic  one. 


CHROMATIC  INTERFERENCE. 


53 


Plates  of  amorphous  or  isometric  minerals  often  show  the  phenom- 
ena of  becoming  partially  or  completely  light  between  crossed  nicols. 
Such  anomalies  are  the  results  of  internal  strains  produced  either  by 
inclusions  of  gases  or  fluids  which  exert  a  pressure  on  their  surround- 
ing walls,  or  by  solid  bodies  which  in  contracting  exert  a  tensile  strain 
on  the  adjacent  parts  of  the  inclosing  mineral ;  or  they  depend  on  con- 
ditions connected  with  the  genesis  of  the  mineral,  that  is,  with  its 
molecular  structure.  Such  phenomena  are  distinguished  from  regular 
double  refraction  by  the  fact  that  the  appearance  is  not  generally  alike 
in  all  parts  of  the  plate,  but  differs  from  place  to  place.  Such  double 
refraction  is  called  an  optical  anomaly. 

Thin  Plates  of  Doubly  Refracting  Minerals  in  Parallel  Polarized 

Light. 

If  a  transparent  plate  of  a  doubly  refracting  mineral,  which  is  not 
cut  at  right  angles  to  an  optic  axis,  is  placed  between  the  polarizer  and 
analyzer  when  their  principal  sections  make  any  angle  whatever  with 
one  another,  it  generally  gives  rise  to  phenomena  of  chromatic  inter- 
ference. Beginning  with  the  simplest  case,  suppose  that  the  plate  has 
parallel  faces  and  is  everywhere  of  the  same  thickness,  that  the  rays 
fall  at  right  angles  to  it,  and  consequently  traverse  equal  thicknesses 
at  all  points ;  that  the  light  is  homogeneous,  and  that  the  principal  sec- 
tions of  the  polarizer  and  analyzer  make  an  angle  >0°<  90°  with  one 
another.  Upon  this  supposition,  a  ray  (Fig. 
17)  which  strikes  the  plate  at  a  is  separated 
into  two  rays  which  traverse  the  plate  in 
like  directions,  but  with  vibrations  at  right 
angles  to  one  another  and  with  different  ve- 

c5 

locities.  Upon  egress  at  the  point  J,  they 
pass  in  to  air  again  without  deflection  and  advance  parallel  to  each  other; 
but  since  the  velocity  is  different  for  each  ray  within  the  plate,  then  at  5 
one  ray  must  have  advanced  a  certain  number  of  wave-lengths  ahead  of 
the  other.  The  rays  are  therefore  in  different  phases  of  vibration,  and 
retain  this  difference  of  phase  on  their  way  through  the  air.  One  of 
the  rays,  the  extraordinary,  vibrates  parallel  to  the  principal  optic 
section  of  the  plate,  the  ordinary  ray  vibrates  at  right  angles  to  this 
principal  optic  plane,  and  these  directions  of  vibration  do  not  change 
on  their  passage  into  air. 

Let  Fig.  18  lie  in  the  plane  of  the  doubly  refracting  plate,  and  let 
the  projection  of  the  principal  section  of  the  polarizer  on  this  plane 
be  PP»  that  of  the  principal  section  of  the  analyzer  be  AA^  0  be 


.  I? 


54 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


the  angle  between  them,  HH1  be  the  principal  section  of  the  platey 
and  p  the  angle  this  makes  with  PPa  and  OM  =  i  be  the  amplitude 
of  vibration  of  the  ray  coming  from  the  polarizer  which  vibrates 
parallel  to  PPr  Since  the  plane  of  vibration  of  the  ray  neither  coin- 
cides with  the  principal  section  of  the  plate  nor  is  at  right  angles  to 


Fig.  18 


it,  then,  according  to  the  parallelogram  of  forces,  it  will  be  divided 
into  two  rays,  an  ordinary  ray  vibrating  at  right  angles  to  HH^  with 
the  intensity  OL  =  sin  p,  and  an  extraordinary  ray  vibrating  parallel 
to  HHv  with  the  intensity  ON  =  cos  p,  i  being  taken  as  unity. 

If  the  velocity  of  the  ordinary  ray  within  the  plate  is  c0  and  that  of 

the  extraordinary  ray  ce,  and  the  thickness  of  the  plate  d,  then  -  =  o 

Co 

and  -  —  e  are  the  times  in  which  0  and  E  traverse  the  plate.     The 

** 

vibrations  of  a  particle  of  ether  about  its  point  of  equilibrium  follow 
the  laws  for  the  motion  of  a  pendulum.  The  velocity  of  vibration 
is  =  0  at  the  moment  of  its  greatest  distance  from  the  position  of 
equilibrium  ;  it  increases  with  its  approach  to  this,  and  reaches  its 
maximum  the  moment  when  this  point  is  passed.  This  maximum  is 
proportional  to  the  amplitude  of  vibration  (the  intensity).  If  t  de- 
notes the  time  which  has  elapsed  since  the  particle  of  ether  was  at  the 
greatest  distance  from  its  point  of  equilibrium.  T  the  duration  of  an 
oscillation  (that  is,  the  time  which  the  particle  of  ether  takes  to  travel 
from  one  position  of  maximum  elongation  to  the  other  and  back  again), 
then  the  velocity  of  vibration  of  a  particle  of  ether  in  the  path  of  the 

ordinary  ray  at  its  entrance  into  the  plate  .—  sin  p  sin  2?r  ™,  and  that 
of  the  extraordinary  ray  =  cos  p  sin  ^n  ^.  Upon  their  egress  from 


CHR  OMA  TIC  INTERFERENCE.  55 

the  plate  the  velocity  of  vibrations  of  0  and  E,  since  they  advance 
more  slowly  in  the  plate  than  in  air,  are  respectively 


V0  =  sin  p  sin  vn\—iYr 
ve  =  cos  p  sin  £ 

If  the  homogeneous  light  used  has  in  air  the  wave-length  A,  the  time 
of  its  vibration  Tt  and  the  velocity  of  transmission  F,  then  T  =  -^ 
and  the  above  equations  become 

(t        oV 
v0  =  sin  p  sin  *&\Tft ~Y~ 

ve  =  cos  p  sin  Sm-af  ~~  ~T~J- 

Upon  its  passage  into  the  analyzer  the  ordinary  ray  furnishes  one 
component  vibrating  at  right  angles  to  the  principal  axis  AA^  hav- 
ing the  intensity  LQ,  which  is  removed  through  total  reflection  in 
the  analyzer,  and  a  component  vibrating  in  the  principal  section  of  the 
analyzer,  with  the  intensity 

OQ  =  OL  sin  (0  —  p)  =  sin  p  sin  (0  —  p). 

In  the  same  manner  the  extraordinary  ray  upon  its  entrance  into  the 
analyzer  separates  into  one  component  which  disappears  through  total 
reflection  and  vibrates  at  right  angles  to  AA^  with  the  intensity  NR, 
and  one  vibrating  parallel  to  AA^  with  the  intensity 

OR  —  ON  cos  (0  —  p)  —  cos  p  cos  (0  —  p). 

Since  both  rays  traversing  the  analyzer  are  extraordinary,  they  have 
the  same  velocity  of  transmission,  and  their  intensities,  being  derived 
from  the  ordinary  and  extraordinary  ray  of  the  plate,  are  respectively 


.     ..  ,  ,        oV 

-  sin  p  sin  (0  —  p)  sm  2yf\-m  —  --T— 


z. 


It        eV\ 

cos  p  cos  (0  —  p)  sin  2?r(-™  -      —  ). 

^-/  A I 


In  the  doubly  refracting  plate  the  rays  did  not  interfere,  since  their 
planes  of  vibration  stood  at  right  angles  to  one  another;  in  the  analy- 
zer they  have  the  same  plane  of  vibration  and  must  therefore  form 
an  interference  ray,  whose  intensity  must  be  equal  to  the  sum  of  the 


56  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

intensities  of  the  rays  producing  it.    Since  OQ  and  OR  stand  in  oppo- 
site sense  to  one  another,  we  have  for  the  interference  ray 


=IV   —  Iv   —  cospcos(0  — 


—  sin  p  sin  (0  —  p)  sin  %n(  ™  --  j-J 

which  may  be  reduced  to  the  form 

(0  _£\  Y 
F  =  cos3  0  +  sin  2p  sin  2  (0  —  p)  sin2  TTV  --  ^—  .     .     .  (I) 

This  expression  shows  that  in  the  general  case  the  intensity  of  the  in- 
terference ray  emerging  from  the  analyzer  is  composed  of  two  factors, 
one  of  which,  cos2  0,  is  independent  of  the  wave-length  and  only  varies 
with  the  inclination  of  the  principal  sections  of  the  polarizer  and  ana- 
lyzer. For  the  relation  which  is  almost  exclusively  used  in  practice, 
namely,  the  crossed  position  of  the  polarizer  and  analyzer,  0  =90°  ; 
therefore  cos2  0  =  0  and  the  equation  becomes 

(0  _  e\  Y 
P  =  sin  2p  sin  2(0  —  p)  sin2  n-  —  ^r 

or  P  =  sin2  2p  sin2  it-  -  ~  —  . 

A 

The  intensity  of  light  between  crossed  nicols  shown  by  a  doubly 
refracting  plate  which  is  not  cut  at  right  angles  to  an  optic  axis  is  pri- 
rnarily  dependent  on  the  quantity  sin2  2p,  that  is,  on  the  inclination  of 
the  principal  optic  section  of  the  plate  with  reference  to  the  princi- 
pal sections  of  the  polarizer  and  analyzer.  The  value  of  P  is  a  mini- 
mum. and  becomes  zero  when  sin2  2p  =  0,  that  is,  every  time  that 
the  principal  optic  section  of  the  plate  coincides  with  the  principal 
section  of  the  polarizer  (p  =  0)  or  with  that  of  the  analyzer  (p=  90°). 

Upon  rotating  the  plate  360°  in  its  plane,  or  for  a  complete  rota- 
tion  of  the  stage  of  the  microscope,  this  coincidence  occurs  four  times, 
from  which  is  derived  the  rule  that  doubly  refracting  plates  become 
dark  four  times  during  a  complete  rotation  between  crossed  nicols,  the 
positions  of  darkness  occurring  every  90°  from  one  another. 

A  maximum  of  brightness  (P  =  max)  must  occur  when  sin2  2p  =  1, 
that  is,  when  the  principal  section  of  the  plate  is  inclined  45°  to  the 
principal  sections  of  the  polarizer  and  analyzer.  Thus  if  a  doubly 
refracting  plate  is  set  at  a  position  of  darkness  in  homogeneous  light, 


CHROMA  TIC  INTERFERENCE.  57 

then  by  rotating  it  an  illumination  will  set  in  which  will  increase  with 
the  rotation  until  it  reaches  45°,  beyond  which  it  will  diminish,  be- 
coming =  0  when  the  angle  of  rotation  =:  90°.  For  every  position  of 
darkness  the  principal  optic  section  of  the  plate  or  a  plane  at  right 
angles  to  it  is  parallel  to  the  principal  section  of  the  polarizer,  and  this 
observation  furnishes  a  means  not  only  of  distinguishing  anisotropic 
from  isotropic  plates,  but  also  of  determining  the  position  of  the  axes 
of  greatest  and  least  elasticity  in  the  plate. 

f0  —  e\  y 
The  brightness  of  the  plate  is  further  dependent  on  sin3  n- — ^-— , 

that  is,  on  the  color  (wave-length)  of  the  light  used ;  on  o  —  0,  that  is, 
on  the  difference  of  phase  of  the  two  rays  traversing  the  plate,  conse- 
quently on  the  difference  between  the  axes  of  greatest  and  least 
•elasticity  in  the  plate,  and  its  orientation  in  the  crystal ;  and  on  its 

thickness,  since  o  —  —  and  e  —  — .     The  quantity  sin2  it- — ^ be- 

co  Ce  A 

(0  _  e\  Y 

comes  =  0  when   -    — ^— - —  is  a  whole  number,  that  is,  when  one  ray 

precedes  the  other  by  a  number  of  whole  wave-lengths.     On  the  other 

(o-e)V  .  (o-e)V       2^  +  1 

liand,  sin  n ~  -  is  a  maximum  when 5-^ —  =  -        — ,  that  is, 

A  A.  Z 

when  one  ray  precedes  the  other  by  an  uneven  number  of  half  wave- 
lengths. Therefore  a  plate  in  homogeneous  light  is  dark  in  every  posi- 
tion between  crossed  nicols.when  the  difference  of  phase  between  the 
two  rays  is  measured  by  whole  wave-lengths,  and  it  has  a  maximum 
brightness  for  every  position  in  which  this  difference  of  phase  is 
measured  by  unequal  half  wave-lengths. 

If  in  equation  (I)  we  suppose  0  >  0  <  90°  and  then  increase  it  by 
90°,  the  expression  becomes 

/*  =  sin2  0  —  sin  2p  sin  2(0  —  p)  sin2  n-—^—.     .     (II) 

This  expression  added  to  equation  (I)  reduces  the  second  member  to  1, 
from  which  it  follows  that  a  rotation  of  the  analyzer  90°  to  the  polar- 
izer reverses  the  phenomena.  What  was  dark  between  crossed  nicols 
must  be  light  with  parallel  nicols. 

If  the  observations  are  made  in  white  light,  the  phenomena  may 
also  be  explained  by  equation  (I),  if  it  is  remembered  that  white  light 
is  composed  of  innumerable  kinds  of  homogeneous  light  of  different 
wave-lengths. 


58  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

The  first  part  of  the  expression  in  equation  (I)  is  independent  of 
the  wave-lengths.  In  the  second  part  of  the  expression, 

(o  -  e)  V 
sin  2p  sin  2(0  —  p)  sin2  n — — , 

sin  2p  sin  2(0  —  p)  is  influenced  by  the  fact  that  the  principal  sections 
of  the  plate  do  not  generally  fall  together  for  different  kinds  of  light. 
But  the  differences  due  to  dispersion  are  for  the  most  part  so  small  that 
they  may  be  neglected.  The  part  of  the  expression  chiefly  affected  is 

fo #)V 

sjn»  n\ —2 — .     And  indeed  all  those  rays  must  disappear  from  the 

A, 

(0  _  e\  y 

white  light  for  which ^—  -  =  n,  when  n  signifies  any  whole  num- 
ber, while  all  those  rays  will  contribute  to  the  illumination  of  the  plate 

(O  —  $\Y 

for  which .>— —  is  a  fraction.     The  plate  will  therefore  appear  col- 

A, 

ored  in  every  case,  and  the  color  will  be  composed  of  those  kinds  of 

(0—e)V       2?i  +  l 
light  for  which T--  —  ~ — ,  or  approach  nearest  to  this  value. 

The  quantities  sin  2/>  and  sin  2(0  —  p)  do  not  influence  the  color 
in  any  way,  but  only  the  intensity  of  the  color.  Therefore  the  color 
shown  by  a  plate  in  polarized  light  does  not  change  in  kind  during  a 
rotation,  but  only  in  intensity.  If  we  again  assume  the  case  which 
occurs  almost  exclusively  in  practice,  namely,  that  the  nicols  are 
crossed,  we  have,  in  equation  (I), 

(0  _  e\  Y 
cos2  0  =  0     and     P  =  sin  2p  sin  2(0  —  p)  sin3  n- — - - —  2i^ 

when  2i^  expresses  the  sum  of  the  endless  number  of  expressions 
which  correspond  to  all  values  of  A. 

The  discussion  of  this  equation  pursued  in  the  same  manner  as  for 
that  of  homogeneous  light  leads  to  the  rule  that  doubly  refracting  plates, 
not  cut  at  right  angles  to  an  optic  axis,  generally  show  an  interference 
color  in  parallel  polarized  white  light,  which  is  dependent  on  their 
thickness  ;  on  the  position  of  the  plates  in  the  crystal,  and  on  the  rela- 
tive size  of  the  axes  of  elasticity;  or  on  the  indices  of  refraction  of,  the 
substance.  The  intensity  of  this  color  depends  on  the  inclination  of 
the  principal  section  of  the  plates  to  the  principal  sections  of  the 
polarizer  and  analyzer,  it  reaches  a  minimum  four  times  in  a  com- 
plete rotation  (the  plate  is  dark)  when  Ms  inclination  is  0°  and  90° : 


NEWTON'S  COLORS.  59 

it  appears  at  a  maximum  four  times  when  the  inclination  is  45°. 
For  the  parallel  position  of  the  principal  sections  of  the  polarizer  and 
analyzer  the  complementary  phenomena  appear :  in  what  is  the  dark 
position  between  crossed  nicols  the  plate  is  white  between  parallel 
nicols ;  in  all  other  positions  the  colors  are  complementary  to  what 
they  were  between  crossed  nicols. 

These  interference  colors  of  doubly  refracting  plates  in  polarized 
light  belong  to  the  category  of  Newton's  colors  (of  thin  plates),  and 
such  a  doubly  refracting  plate  will  show  the  same  interference  colors 
as  an  isotropic  plate  of  the  thickness  d,  if  d  =  (o  —  e)  V.  These  inter- 
ference colors  belong  to  the  most  characteristic  phenomena  of  micro- 
scopical investigation,  and  for  a  known  thickness  and  orientation  of  the 
plate  directly  indicate  the  value  of  (o  —  e),  which  is  among  the  con- 
stants  of  every  substance.  It  is  evident  that  for  a  constant  thickness 
and  the  same  substance  the  interference  color  will  be  higher,  as  there 

O  ? 

is  a  greater  difference  between  the  two  axes  of  elasticity  to  which  the 
vibrations  of  the  rays  are  parallel.  Therefore  optically  uniaxial  bodies, 
other  things  being  equal,  must  give  the  highest  interference  colors  in 
sections  parallel  to  the  optic  axis,  arid  optically  biaxial  bodies  in  sec- 
tions at  right  angles  to  the  axis  of  mean  elasticity.  The  interference 
colors  must  diminish  as  the  plate  is  cut  more  nearly  perpendicular  to 
an  optic  axis,  and  the  colored  interference  ceases  when  the  light 
traverses  the  plate  exactly  parallel  to  an  optic  axis. 

Newton  determined  the  order  of  succession  of  the  interference 
colors  shown  by  thin  plates  of  increasing  thickness  and  arranged  them 
in  a  color-scale  bearing  his  name.  It  will  be  seen  from  the  accompany- 
ing table  that  certain  tones  of  color  recur  periodically ;  the  colors  which 
lie  between  two  analogous  tones  are  called  an  "  order."  A  knowledge  of 
this  color  scale  greatly  facilitates  the  estimation  of  the  amount  of  double 
refraction  peculiar  to  a  particular  mineral,  and  is  absolutely  necessary 
in  the  determination  of  the  optical  characters  of  a  mineral  cross-section. 

behavior  of  Doubly  Refracting  Plates  cut  at  Right  Angles  to  an 
Optic  Axis  in  Polarized  Light. 

In  every  direction  at  right  angles  to  an  optic  axis  in  a  doubly  re- 
fracting mineral  the  elasticity  of  the  ether  is  the  fame,  and  for  all  rays 
travelling  exactly  parallel  to  this  axis  the  mineral  should  behave  like  an 
isotropic  medium.  If  a  section  at  right  angles  to  an  optic  axis  be 
examined  between  crossed  or  parallel  nicols  in  parallel,  homogeneous 
light,  then  in  the  first  case  one  would  expect  it  to  remain  dark  for  a 


60 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS, 


NEWTON'S  COLOR-SCALE  ACCORDING  TO  QUINCKE.' 


No. 

Millionths 
of 
Millimeters. 

Interference  Color 
betweeen 
Crossed  Nicols. 

Interference  Color 
between 
Parallel  Nicols. 

1 

0 

Black. 

Bright  white.                         "1 

2 

40 

Iron  -gray. 

White. 

3 

97 

Lavender-gray. 

Yellowish  white. 

4 

158 

Grayish  blue. 

Brownish  white. 

5 

218 

Clearer  gray. 

Brownish  yellow. 

6 

234 

Greenish  white. 

Brown. 

Ky 

7 

259            Almost  pure  white. 

Light  red. 

S|- 

8 

267 

Yellowish  white. 

Carmine-red. 

§5. 

9 

275 

Pale  straw-yellow. 

Dark  reddish  brown. 

? 

10 

281 

Straw-yellow. 

Deep  violet. 

i 

11 
12 

306 
332 

Light  yellow. 
Bright  yellow. 

Indigo. 
Blue. 

j? 

13 

430 

Brownish  yellow. 

Gray-blue. 

14 

505 

Reddish  orange. 

Bluish  green. 

15 

536 

Red. 

Pale  green. 

16 

551 

Deep  red. 

Yellowish  green. 

17 

565 

Purple. 

Lighter  green. 

18 

575 

Violet. 

Greenish  yellow. 

19 

589 

Indigo. 

Golden  yellow. 

20 

664 

Blue  (sky-blue). 

Orange. 

21 

728 

Greenish  blue. 

Brownish  orange. 

22 

747 

Green. 

Light  carmine-red. 

§ 

23 

24 

826 
843 

Lighter  green. 
Yellowish  green. 

Purplish  red. 
Violet-purple. 

P* 

25 

866 

Greenish  yellow.           .p 

Violet. 

s^ 

26 

910 

Pure  yellow. 

Indigo. 

r* 

27 

948 

Orange. 

Dark  blue. 

28 

998 

Bright  orange-red. 

Greenish  blue. 

29 

1101 

Dark  violet-red. 

Green. 

30 

1128 

Light  bluish  violet. 

Yellowish  green.                   1 

31 
32 

1151 
1258 

Indigo. 
Greenish  blue. 

Impure  yellow. 
Flesh  -colored. 

| 

33 
34 

1334 
1376 

Sea-green.  % 
Brilliant  green. 

Brownish  red. 
Violet. 

«**• 

si 

>•  -». 

35            1426 

Greenish  yellow. 

Grayish  blue. 

36            1495 

Flesh-colored. 

Sea-green. 

1 

37            1534 

Carmine-red. 

Green. 

38 

1621 

Dull  purple. 

Dull  sea-green. 

39 

1652 

Violet-gray. 

Yellowish  green.                    ]     ^ 

40 

1682 

Grayish  blue. 

Greenish  yellow. 

f 

41 

1711 

Dull  sea-green. 

Yellowish  gray. 

5 

42 

1744 

Bluish  green. 

Lilac. 

43 

1811 

Light  green. 

Carmine.                                        O 

44 
45 

1927 
2007 

Light  greenish  gray. 
Whitish  gray. 

Grayish  red.                                 J 
Bluish  gray.                           J     r* 

*  Ueber  Newton 'sche  Farbenringe  und  totale  Reflexion  des  Lichtes  bei  Metallen. 
Pogg.  Ann.  1866.  CXXIX.  177. 


INTERNAL  CONICAL  REFRACTION.  61 


complete  rotation  in  its  own  plane,  and  in  the  second  case  to  remain 
illuminated.  The  actual  appearance,  however,  is  different  for  an  optic- 
ally uniaxial  and  biaxial  substance.  In  tetragonal  and  hexagonal  crys- 
tals the  principal  axis  is  also  the  optic  axis,  that  is,  the  direction  of 
single  refraction  for  light  of  every  color ;  and  a  basal  section  of  such 
minerals  for  perpendicular  incidence  in  parallel  light  acts  the  same  for 
every  color  and  every  color  combination,  consequently  for  white  light 
it  acts  like  an  isotropic  body.  The  distinction  of  such  a  basal  section 
of  a  uniaxial  mineral  from  a  section  of  an  isotropic  substance  is  made 
by  investigation  in  polarized  light  which  is  not  parallel. 

In  the  orthorhombic,  monoclinic,  and  triclinic  systems  the  optic  axes 
no  longer  coincide  with  the  axes  of  symmetry ;  they  therefore  suffer  a 
dispersion,  and,  strictly  speaking,  there  can  no  longer  be  any  section 
which  shall  be  perpendicular  to  an  optic  axis  for  two  different  colors 
at  the  same  time.  Consequently  it  is  not  to  be  expected  that  such  a 
section  would  behave  like  an  isotropic  plate,  but  rather  that  in  every 
position  between  two  nicols,  making  any  angle  whatever  with  one 
another,  such  a  plate  in  parallel  white  light  would  be  illuminated  by  a 
color  approaching  the  lowest  tints  of  Newton's  color  scale.  But  even 
with  the  use  of  homogeneous  light  thin  sections  of  a  biaxial  mineral 
cut  at  right  /angles  to  an  optic  axis  are  riot  dark,  but  light,  and  they 
are  light  in  every  position  during  a  complete  rotation  in  their  own 
plane.  This  apparent  anomaly  as  recently  shown  by  E.  Kalkowsky 
(Z.  X.  1884,  ix.  486-497)  is  the  necessary  consequence  of  the  fact  that 
the  optic  axis  of  biaxial  bodies  are  axes  of  internal  conical  refraction. 
A  ray  of  light  falling  parallel  to  an  optic  axis  on  a  biaxial  plate  cut  at 
right  angles  to  this  axis  is  divided  within  the  same  into  an  infinite 
number  of  rays  which  lie  on  the  surface  of  a  cone  and  are  polarized  in 
all  directions.  They  thus  emerge  as  a  cylinder  of  rays  in  which  each 
ray  vibrates  in  a  different  azimuth  from  the  rest ;  between  crossed 
nicols,  then,  such  a  plate  must  show  the  same  .illumination  in  all  posi- 
tions. This  phenomenon  distinguishes  sections  in  this  direction  when 
they  are  sufficiently  thin  from  those  of  any  other  direction. 

Behavior  of  Several  Doubly  Refracting  Plates  lying  upon  one  another 

in  Polarized  Light. 

If  two  doubly  refracting  plates  overlie  one  another,  the  resultant 
phenomena  in  polarized  light  depend  on  the  inclination  of  the  princi- 
pal sections  of  the  polarizer  and  analyzer  to  one  another  and  to  the 
principal  sections  of  the  plates,  as  may  be  seen  by  a  further  application 


62  PUYSIOGRA1IY  OF  THE  ROCK-MAKING  MINERALS, 

of  the  methods  employed  in  discussing  Fig.  18.  By  using  white  light 
there  will  be  an  interference  color  whose  height  is  dependent  on  the 
sum  of  the  thicknesses  of  both  plates  and  the  sum  of  the  differences  of 
phase  attained  by  the  rays  in  both  plates,  and  whose  intensity  is  deter- 
mined by  the  inclination  of  the  principal  sections  of  both  plates  to  one 
another  and  to  those  of  the  nicols. 

If  the  principal  sections  of  the  two  plates  are  perpendicular  or  par- 
allel to  one  another,  then  the  system  of  the  two  plates  with  respect  to 
the  four  occurrences  of  the  extinction  of  light  between  crossed  nicols 
will  act  just  as  a  single  plate.  The  interference  color,  which  appears 
when  the  principal  sections  of  the  plates  are  other  than  at  90°  or  0°  to 
those  of  the  nicols,  will  rise  in  comparison  with  the  interference  colors  of 
each  single  plate  if  with  the  parallel  position  of  their  principal  sections 
to  one  another  equivalent  axes  of  elasticity  fall  together ;  or  with  the 
crossed  position  if  un equivalent  axes  of  elasticity  fall  together.  On  the 
other  hand,  it  will  be  lowered  when  the  opposite  conditions  exist.  If 
one  therefore  knows  the  optical  character  of  one  plate,  then  by  observ- 
ing the  rising  or  sinking  of  the  interference  color  upon  the  insertion 
of  a  second  plate  in  parallel  or  right-angled  position  of  the  principal 
sections  the  relative  value  of  the  axes  of  elasticity  in  this  second  plate 
may  be  determined. 

Plates  of  Anisotropic  Twinned  Crystals  in  Polarized  Light. 

In  sections  of  twinned  crystals  the  parts  belonging  to  each  individual 
must  in  general  behave  differently  in  polarized  light,  since  their  axes  of 
elasticity  are  differently  oriented  with  respect  to  the  principal  section 
of  ihe  nicols.  The  position  of  darkness  for  each  lamella  will  naturally 
be  reached  when  its  axes  of  greatest  and  least  elasticity  coincide  with 
the  principal  sections  of  the  polarizer  and  analyzer.  The  application 
of  the  rules  previously  given  for  the  behavior  of  doubly  refracting 
lamellae  in  polarized  light  shows  that  for  certain  positions  between 
crossed  nicols  the  lamellae  belonging  to  a  twin  must  appear  equally 
bright,  and  for  sufficiently  thin  lamellae  must  also  be  of  the  same  color. 
This  happens  when  the  principal  sections  of  each  half  of  the  twin  are 
equally  inclined  on  opposite  sides  of  the  principal  sections  of  the 
nicols. 

If  the  section  through  a  system  of  twinned  lamellae  is  not  perpen- 
dicular to  their  composition-plane,  then  there  must  be  strips  between 
each  two  adjacent  lamellae  which  consist  of  wedges  of  both  lamellae 
overlapping  one  another.  The  behavior  of  these  strips  between  crossed 


STA  UROSCOPIC  METHODS.  63 

nicols  will  be  understood  by  considering  their  action  as  twins  and  also 
as  superimposed  plates. 

.Stauroscopie  Methods  for  determining  the  Direction  of  Extinction 
in  Doubly  Refracting  Plates. 

Since  the  determination  of  the  direction  of  the  extinction  of  light 
in  a  doubly  refracting  plate  furnishes  criteria  for  the  recognition  of  the 
position  of  the  axes  of  elasticity  in  the  mineral  with  respect  to  the 
crystal  axes,  and  consequently  for  the  discovery  of  the  crystal  system, 
it  is  one  of  the  most  important  determinative  expedients.  Now  the 
eye  is  relatively  insensible  to  small  variations  in  the  brightness  of  light, 
and  it  is  evident  that  the  readings  of  the  positions  of  greatest  darkness 
•of  doubly  refracting  plates  when  using  white  light  may  differ  consider- 
ably. It  is  more  correctly  effected  by  using  monochromatic  light,  but 
this  is  not  convenient,  and  numerous  methods  have  been  sought  which 
would  furnish  greater  exactness  in  the  adjustment  to  the  maximum  of 
darkness  without  using  monochromatic  light.  A  calcite  plate  cut  at  right 
angles  to  the  optic  axis  was  employed  by  Kobell  (Pogg.  Ann.  1855, 
jccv.  p.  320).  Placed  between  the  object  and  the  analyzer,  it  shows  an 
interference  figure  consisting  of  a  dark  cross  and  a  number  of  concen- 
tric isochromatic  rings  when  the  principal  sections  of  the  object  and 
of  the  nicols  coincide,  the  interference  figure  being  distorted,  when 
they  are  not  coincident.  Such  an  instrument  is  called  a  stauroscope. 
Erezina  improved  the  sensitiveness  of  this  method  by  substituting  for 
the  single  calcite  plate  a  system  of  two  plates  cut  nearly  at  right  angles 
to  the  axis.  If  the  calcite  plate  is  placed  between  the  ocular  of  a 
microscope  and  the  nicol  above  it,  it  becomes  a  stauroscope. 

A  more  exact  method  for  detecting  the  direction  of  extinction  in 
doubly  refracting  plates  than  the  use  of  maximum  darkness  is  the 
use  of  a  particular  color.  This  is  most  conveniently  accomplished  by 
inserting  between  the  crossed  nicols  a  quartz  plate  cut  parallel  to  the 
axis,  and  of  such  a  thickness  that  it  will  show  a  violent  interference 
color  (18  of  Newton's  color-scale) ;  if  the  axis  of  the  quartz  plate  be 
set  at  45°  to  the  principal  sections  of  the  nicols,  the  whole  field  will  be 
equally  colored  violet.  If  a  doubly  refracting  plate  be  placed  so  as  to 
cover  part  of  the  field  only,  it  will  appear  of  a  different  color  from  the 
violet,  because  the  difference  of  phase  for  the  rays  emerging  from  the 
plate  is  added  to  that  derived  from  the  quartz.  If  now  the  plate  is 
rotated  till  its  axis  of  greatest  and  least  elasticity  coincide  with  the  prin- 
cipal sections  of  the  nicols,  a  dissection  of  the  light  by  the  plate  will 


64  PHYSIOGRAPHY  OF  THE  ROL'K-XAKISG  MINERAL*. 

no  longer  take  place,  and  the  plate  will  be  colored  the  same  as  the 
quartz  plate.  This  adjustment  to  the  color  of  the  quartz  plate  is 
extremely  sensitive  for  colorless  or  very  slightly  colored  minerals. 

E.  Bertrand  inserted  in  the  ocular  of  the  microscope  a  quartz  plate 
composed  of  two  pairs  of  right-  and  left-handed  quartzes  of  the  same 
thickness  which  are  cut  perpendicular  to  the  axis  and  cemented  to- 
gether so  that  each  pair  occupies  opposite  quadrants 
(Fig.  19).  This  plate  is  set  in  the  ocular  so  that 
the  lines  of  contact  between  the  four  parts,  which 
appear  as  two  dark  lines  at  right  angles  to  one 
another,  shall  be  exactly  parallel  to  the  principal 
sections  of  the  nicols.  When  the  nicols  are  crossed 
all  four  quartz  quadrants  present  the  same  tint  of 
color.  Upon  introducing  a  doubly  refracting  plate 
on  the  stage  of  the  microscope,  the  opposite  sectors 
of  the  plate  are  similarly  colored  and  the  adjacent  ones  dissimilarly 
colored.  They  all  become  alike  when  the  principal  sections  of  the 
plate  are  made  parallel  to  those  of  the  nicols.  The  Bertrand  ocular 
undoubtedly  furnishes  the  most  exact  stauroscopic  determination,  and 
is  in  the  most  convenient  form. 


Determination  of  the  Relative    Value  of  Both  Axes  of  Elasticity 
in  a  Doubly  Refracting  Plate. 

In  the  microscopical  determination  of  minerals  it  is  frequently  nec- 
essary to  determine  which  of  the  directions  of  extinction  in  a  plate 
corresponds  to  the  axis  of  greatest  elasticity,  and  which  to  that  of  least 
elasticity.  This  problem,  called  the  determination  of  the  optical  char- 
acter, is  solved  by  means  of  a  plate  of  known  character. 

When  the  position  of  the  axes  of  elasticity  in  the  plate  is  deter- 
mined the  plate  is  rotated  so  that  its  principal  sections  make  angles  of 
45  with  those  of  the  crossed  nicols  ;  the  interference  color  is  thus  at 
its  maximum.  A  thin  mica  plate  is  then  placed  either  in  the  lower 
end  of  the  tube  of  the  microscope  or  between  the  ocular  and  upper 
analyzer  in  such  a  position  that  its  previously  determined  axes  of  elastic- 
ity are  parallel  to  those  of  the  plate  under  investigation.  The  differ- 
ence of  phase  of  the  rays  will  then  be  increased  when  equivalent  axes 
of  elasticity  in  the  two  plates  fall  together,  and  will  be  diminished 
when  uneqnivalent  axes  of  elasticity  cover  one  another.  In  the  first 
case  the  mica  plate  acts  like  a  thickening  of  the  plate,  and  the  inter- 
ference color  must  rise  in  the  scale ;  in  the  second  case  it  acts  like  a 


QUARTZ  WEDGE.  65 

thinning  of  the  plate,  and  the  interference  color  most  fall.  Since  in 
the  mica  plate  the  axis  of  smallest  elasticitj*  lies  parallel  to  the  plane  of 
its  optic  axes,  which  is  easily  determined,  the  observation  leads  directly 
to  the  desired  result.  The  same  result  may  be  obtained  by  using  a 
quartz  plate  cut  parallel  to  its  axis. 

For  strongly  doubly  refracting  plates  (for  example  the  microscopic 
zircons  of  rocks)  it  is  well  to  use  a  quartz  wedge  to  determine  the 
relative  values  of  the  axes  of  elasticity.  Such  a  quartz  wedge  is  cut  so 
that  one  of  its  faces  is  exactly  parallel  to  the  principal  axis  (optic  axis, 
axis  of  least  elasticity),  while  the  other  face  makes  a  very  small  angle 
with  it.  The  long  side  of  the  wedge  gives  the  direction  of  the  princi- 
pal axis.  If  the  wedge  is  pushed  between  crossed  nicols  so  that  its 
principal  axis  is  inclined  45°  to  the  principal  sections  of  the  nicols, 
then  the  whole  series  of  Newton's  colors  from  iron-gray  of  the  first 
order  through  the  second  or  third  order  appears  in  a  succession  of  bands 
if  one  moves  the  wedge  forward  toward  its  thin  edge :  when  moved  in 
the  opposite  direction  the  succession  is  reversed.  If  at  the  same  time 
the  plate  to  be  investigated  lies  with  its  principal  section  inclined  at 
45°  to  those  <3f  the  nicols,  the  color  of  the  quartz  wedge  will  be 
changed  in  the  place  where  it  covers  the  plate,  and  the  new  color  will 
be  that  of  a  thicker  part  of  the  quartz  wedge  when  the  axis  of  smallest 
elasticity  of  the  plate  lies  parallel  to  the  axis  of  the  wedge.  On  the 
other  hand,  the  new  color  will  correspond  to  that  of  a  thinner  part  of 
the  wedge  when  the  axis  of  greatest  elasticity  of  the  plate  is  parallel  to 
the  axis  of  the  wedge. 

Such  a  quartz  wedge  also  serves  to  determine  the  order  of  the  inter- 
ference color  of  a  doubly  refracting  plate.  Suppose  the  plate  shows 
red,  and  that  its  principal  section  is  turned  45°  to  those  of  the  nicols. 
If  the  quartz  wedge  is  pushed  between  the  ocular  and  upper  nicol 
with  its  thin  edge  forward,  so  that  its  axis  of  smallest  elasticity  is  par- 
allel to  the  axis  of  greatest  elasticity  in  the  plate,  then  the  interference 
color  must  descend  in  the  scale  as  thicker  parts  of  the  quartz  wedge 
come  to  lie  over  the  plate.  The  plate  then  shows  one  after  another 
the  Xewton  colors  in  descending  order,  until  the  acceleration  of  one 
of  the  rays  in  the  plate  exactly  corresponds  to  the  retardation  of  the 
same  in  the  quartz  wedge.  At  this  instant  it  is  the  same  as  though 
the  plate  were  crossed  by  an  exactly  similar  plate  of  the  same  sub- 
stance. The  plate  must  appear  gray  or  black  according  to  the  strength 
of  its  dispersion  of  color.  If  during  this  operation  the  original  color 
of  the  plate  (red)  recurred  n  times,  then  the  original  color  of  the  plate 
must  have  been  of  the  n  +  1  order. 


66 


PHYSIOGRAPHY  OF  THE  RQCK-AMK1XU 


Determination  of  the   Index  of  Refraction  in  Doubly  Refracting 

Plates. 

Owing  to  the  extreme  thinness  of  the  sections  used  in  microscopi- 
cal investigation,  and  to  the  generally  small  double  refraction  of  the 
rock-making  minerals,  the  same  methods  may  be  used  for  determining 
the  coefficient  of  refraction  which  were  given  for  isotropic  media  in 
thin  plates,  with  no  greater  error  than  the  conditions  of  the  case  neces- 
sarily impose.  Consequently  the  plate  to  be  investigated  must  first 
be  brought  into  its  position  of  darkness  between  crossed  nicols,  and 
after  the  analyzer  has  been  removed,  the  polarizer  being  retained,  the 
index  of  refraction  for  the  ray  vibrating  parallel  to  the  principal  sec- 
tion is  determined  according  to  the  method  given  on  page  28,  the 
position  of  the  plate  of  course  remaining  unchanged.  If  the  plate 
is  then  rotated  90°  in  its  plane,  the  index  of  refraction  for  the  second 
ray  may  be  found  in  the  same  manner.  The  values  found  are  only 
the  principal  indices  of  refraction  of  the  mineral,  when  the  plate  from 
which  they  were  derived  has  been  cut  at  right  angles  to  an  axis  of  elas- 
ticity. The  best  signal  which  can  be  used  is  a  microscopic  photograph 
on  glass  of  a  newspaper  clipping  with  various-sized  type,  or  a  system 
of  crossed  lines.  This  may  be  fastened  with  wax  to  the  lower  end  of 
the  polarizer,  and  the  reduced  image  which  is  projected  above  the  con- 
densing lens  of  the  polarizer  used  to  focus  on.  The  following  table 
gives  the  indices  of  refraction  of  the  most  important  rock-making  min- 
erals, arranged  in  descending  order. 


Anatase     .     . 
Cassiterite 
Zircon 

2.524 
2.029 
1  987 

Axinite     .     .    „ 
Olivine      .     . 
Bronzite 

1.680 
1.678 
1  668 

Dipyre  .  .  .  .  1.554 
Muscovite  .  .  .  1.551 
Quartz  1  551 

Titanite     .     . 
Pyrope      .     . 
Aegerine  . 

1.910 

1.812 

1.808 

Sillimanite     . 
Glaucophane 
Andalusite 

1.660 
1.644 
1  638 

Cordierite  .  .  .  1.542 
Nepheline  .  .  .  1.540 
Albite  1  532 

Almadine  .     . 
Corundum     . 
Grossular  .     . 
Epidote     .     . 
Staurolite 
Vesuvianite   . 
Disthene   .     . 
Spinel  .     .     . 

1.766 
1.764 
1.761 
1.756 
1.753 
.     1.726 
.     1.724 
.     1.717 

Apatite      .     . 
Anthophyllite 
Tourmaline    . 
Actinolite 
Melilite     .     . 
Tremolite 
Dolomite  .     . 
Topaz 

1.637 
1.636 
1.635 
1.629 
1.629 
1.623 
1.622 
1  620 

Sanidine  .  .  .  .  1.524 
Cancrinite  .  .  .  1.515 
Leucite  .  .  .  .  1.508 
Haiiyne  .  .  .  .  1.499 
Sodalite  .  .  .  .  1.488 
Analcite  ....  1.488 
Natrolite  ....  1.480 
Opal  .  1  455 

Zoisite  .     .     . 
Coccolite   .     . 
Hyperstkene  . 

r 

.     1.695 
.     1.690 
.     1.685 

Calcite  .     .    . 
Meionite    .     . 
Chlorite    .    . 

• 

1.601 
1.578 

1.577 

Fluorite  ....  1.435 
Tridymite  .  .  .  1.428 

CONVERGENT  POLARIZED  LIGHT.  67 

d.  Investigation  of  Minerals  in  Convergent  Polarised  Light. 

As  observation  in  parallel  polarized  light  furnishes  the  means  of 
determining  the  position  of  the  axes  of  elasticity  in  a  doubly  refracting 
plate,  and  of  establishing  by  a  number  of  such  determinations  on  plates 
from  different  positions  in  a  crystal  the  orientation  of  the  axes  of 
elasticity  with  respect  to  the  crystal  axes,  and  consequently  the  crystal 
system,  so  observation  in  convergent  polarized  light  serves  to  distin- 
guish uniaxial  crystals  from  biaxial,  to  determine  the  dispersion  and 
the  optical  character,  and  finally  makes  it  possible  to  decide  whether  a 
plate  which  appears  isotropic  in  parallel  polarized  light  belongs  to  an 
isotropic  substance  or  to  an  anisotropic  one  that  has  been  cut  at  right 
angles  to  its  optic  axis. 

Means  of  Observation. — The  Norremberg  polarization  instrument 
is  commonly  used  for  macroscopical  investigations  of  minerals  in  con- 
vergent polarized  light.  But  for  microscopical  investigation  this  is  not 
applicable,  therefore  the  microscope  has  been  arranged  for  observations 
in  convergent  light. 

If  on  the  same  metal  tube  which  holds  the  polarizer  a  strong  con- 
densing lens  or  system  of  lenses  be  screwed,  and  this  be  pushed  as  close 
as  possible  to  the  object  by  raising  the  polarizer,  the  glasses  on  either 
side  of  the  object  being  as  thin  as  possible,  and  if  a  strong  objective 
lens  be  brought  as  close  as  possible  to  the  object,  then  the  plate  under 
investigation  will  be  in  exactly  the  same  condition  as  in  the  ISTorrem- 
berg  apparatus,  and  the  strongly  divergent  bundle  of  rays  which  traverse 
the  plate  will  be  united  by  the  objective  to  form  an  image.  If  the 
•ocular  be  removed  and  the  analyzer  be  in  place,  the  diminished  image 
will  appear  at  a  somewhat  greater  distance  than  before.  It  will  be 
extremely  sharp  and  clear,  but  very  small.  This  method  was  first  pro- 
posed by  A.  v.  Lassaulx. 

In  order  to  obtain  a  larger  image  and  retain  the  cross- wires  which 
are  situated  in  the  ocular,  E.  Bertrand  introduced  into  the  tube  of  the 
microscope  above  the  focus  of  the  objective  a  weak  condensing  lens, 
which  unites  with  the  ocular  of  the  microscope  to  form  a  new  micro- 
scope for  observing  the  image  projected  by  the  objective  system  of 
lenses.  The  field  of  view  is  larger  the  stronger  the  system  of  lenses 
above  the  polarizer  and  in  the  objective  ;  and  in  order  to  prevent  total 
reflection  on  the  layers  of  air  between  the  plate  and  systems  of  lenses 
with  strong  convergence,  it  is  well  to  introduce  between  them  a 
strongly  refracting  fluid,  such  as  almond-oil,  etc.,  which  cannot  injure 
the  lenses  or  their  metal  frames. 


68  PHYSIOGRAPHY  OF  THE  ROCK-MA  KINO  MINERALS. 


Interference  Phenomena  of    Uniaxial    Plates   cut  Perpendicular 
to  the  Axis  in  Convergent  Polarized  light. 

It  has  already  been  pointed  out  that  a  plate  of  a  uniaxial  mineral  cut 
parallel  to  a  basal  plane  behaves  exactly  the  same  in  parallel  polarized 
light  as  a  plate  of  an  isotropic  mineral.  Both  remain  dark  during  a 
complete  rotation  in  their  plane  between  crossed  nicols,  and  are  bright 
between  parallel  nicols.  If  the  parallel  light  is  replaced  by  convergent 
there  will  be  no  change  of  this  phenomena  with  isotropic  plates, 
for  in  no  case  can  there  be  a  separation  or  change  of  the  planes  of 
vibration  of  the  rays  coming  from  the  polarizer.  On  the  other  hand, 
basal  sections  of  uniaxial  crystals  show  polarization  phenomena  in  con- 
vergent light  which  they  do  not  exhibit  in  parallel  light.  We  will 
confine  ourselves  to  an  explanation  of  the  phenomena  which  occur  be- 
tween crossed  and  parallel  nicols  in  convergent  light,  these  being  the 
only  positions  of  practical  importance. 

Let  A  (Fig.  20)  be  the  cross-section  of  a  basal  plate  of  a  tetra- 
gonal or  hexagonal  mineral 
which  is  between  crossed  nicols 
and  is  traversed  by  strongly 
converging  rays  of  homogene- 
ous light.  Let  ccl  be  the  prin- 
A  j  cipal  cry  stall  ographic  axis,  and 

'     consequently    the    optic     axis. 

Then  all  rays  from  the  polar- 
izer entering  the  plate   at  this 
c,  point  with   perpendicular  inci- 

Fie.  so  dence,   and     therefore    passing 

through  it  parallel  to  the   optic 

axis,  will  experience  no  alteration  whatever  ;  when  they  reach  the 
analyzer  they  enter  it  as  ordinary  rays,  and  are  totally  reflected  from 
the  balsam  film  ;  the  middle  of  the  plate  will  then  appear  dark.  A 
ray,^,  on  the  contrary,  upon  entering  the  plate  will  be  separated  into 
two  rays,  im  and  il,  of  which  one  vibrates  in  the  principal  section,  ficc^ 
the  other  at  right  angles  to  it.  In  the  same  way  the  ray  eh  parallel  to 
fi  will  be  divided  into  the  rays  hi  and  M,  and  so  on.  Thus  there  emerge, 
as  at  I,  from  every  point  on  the  plate  two  rays  in  parallel  directions,  an 
ordinary  and  an  extraordinary  ;  one  of  each  of  these  pairs  of  rays  has 
traversed  the  plate  with  a  different  velocity  from  the  other,  and  more- 
over their  paths  in  the  plate  il  and  hi  are  of  different  lengths.  They 


?i  Jt/ 


UNIAXIAL  INTERFERENCE  FIG  URE.  69 

are  thus  at  I  in  different  phases  of  vibration,  and  have  planes  of  vibra- 
tion at  right  angles  to  one  another.  Reaching  the  analyzer,  each  of  two 
such  rajs  splits  up  into  an  ordinary  and  an  extraordinary  component. 
The  ordinary  ones  are  totally  reflected  from  the  balsam  film  ;  the  ex- 
traordinary come  to  an  interference  through  the  difference  of  phase 
which  they  acquired  in  the  plate,  since  they  are  now  reduced  to  the 
same  plane  of  vibration.  The  intensity  of  these  interference  rays  will 
be  expressed  approximately  by  the  formula  on  page  58, 

fo e)V 

P  =  sin  2p  sin  2(0  —  p)  sin2  n r-* — . 


In   this,   as   previously  shown,  - — ~ — .expresses  the  difference  of 

phase  dependent  upon  the  difference  of  the  axes  of  elasticity  in  the  plate, 
or  of  their  reciprocals  ;  upon  the  thickness  of  the  plate,  and  upon  the 
wave-lengths.  If  this  difference  of  phase  is  given  by  (p— e)  V  =  A, 
that  is,  one  wave-length,  then  /2  =  0  and  the  point  I  must  appear  dark 
between  crossed  nicols ;  while  between  I  and  the  focus  of  the  axis  it  is 
light.  Now  since  it  is  evident  that  for  a  plate  of  uniform  thickness 
the  difference  of  phase  must  be  the  same  for  all  rays  which  emerge  at 
the  same  distance,  Z#,  from  the  axis  of  the  plate,  and  have  the  same 
inclination  to  it,  so  there  must  appear  a  continuous  row  of  dark  points 
at  a  distance  lo  from  the  locus  of  the  axis,  that  is,  a  dark  circle  with 
the  radius  lo. 

At  a  somewhat  greater  distance  than  lo  the  difference  of  the  phase 
of  the  rays  which  emerge  with  greater  inclination  to  cct  will  be  >  A, 
because  the  difference  a  —  y  increases  with  the  distance  from  cc^  and 
the  difference  in  the  paths  of  the  rays  within  the  plate  also  increases. 
The  plate  will  be  light  in  such  places,  and  the  maximum  of  illumination 
will  lie  at  that  distance  from  0,  for  which  the  difference  of  phase  of 
the  rays  is  f  A,  for  then 

P  —  sin  2  p  sin  2(0  —  p). 

Naturally  the  same  must  apply  to  all  points  equidistant  from  0,  and 
there  must  be  outside  of  the  dark  ring  whose  radius  is  lo  a  bright  one 
with  a  radius  greater  than  lo. 

For  still  greater  distances  from  o  the  difference  of  phase  will  be 

~ ^    ^  ^  ^'  ^e  bright11688  diminishes  and  reaches  a  minimum 


70  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

±=2,  and  at  this  distance  there  will  be  another  dark 

/L 

ring. 

Proceeding  in  the  same  manner,  it  is  evident  that  a  basal  plate  of  a 
uniaxial  mineral  in  homogeneous  light  between  crossed  nicols  must 
show  a  dark  centre,  and  a  succession  of  light  and  dark  rings.  The 
distance  of  the  dark  rings  apart  depends  on  A,  consequently  it  is  differ- 
ent for  red,  yellow,  and  other  lights  ;  it  depends  on  o  —  e,  that  is,  on  the 
strength  of  the  double  refraction  in  the  plate,  or  on  the  difference  be- 
tween its  indices  of  refraction  GO  and  e,  and  on  the  thickness  of  the  plate, 
with  which  indeed  both  the  length  of  the  path  of  the  rays  and  their 
difference  of  phase  increase.  The  diameter  of  the  rings  is  propor- 
tional to  the  wave-lengths,  and  inversely  proportional  to  the  thickness 
of  the  plate  and  the  strength  of  the  double  refraction.  The  distance 
of  the  rings  from  one  another  decreases  with  the  distance  from  the 
centre  of  the  field.  The  number  of  visible  rings  is  naturally  inversely 
proportional  to  their  diameters. 

Since  for  the  dark  rings  of  such  a  plate  72  =  0,  the  darkness  at  all 
points  of  such  a  ring  is  absolute.  But  for  the  light  rings 

r  =  sin  2p  sin  2(0  —  p)  ; 

their  brightness  therefore  is  not  the  same  at  all  points,  but  is  depen- 
dent on  p,  that  is,  on  the  angle  which  the  principal  section  of  the  plate 
makes  with  the  principal  sections  of  the  nicols. 
Now,  from  a  previous  definition,  the  principal 
section  in  a  uniaxial  crystal  is  the  plane  passing 
through  a  ray  and  the  optic  axis.  Consequently 
for  the  rays  emerging  at  m,  n,  r,  m^  nt,  r.17  (Fig. 
£1,)  of  the  light  ring  IIR,  mo,  no,  or,  etc.,  are 
the  principal  planes,  and  <  mop,nop,  rop,  etc., 
correspond  to  the  angle  p  of  the  formula,  and 
<  moa,  noa,  roa,  etc.,  to  the  angle  (p  —  p.  P  is 
evidently  a  maximum  when  p  =  0  —  p  =  45°, 

and  a  minimum  when  p  =  0,  0  —  p  =  90°,  p  =  90°,  0  —  p  =  0°.  The 
light  ring  has  therefore  a  maximum  of  intensity  at  n  and  n^  and  at  the 
corresponding  points  of  both  the  other  quadrants,  ftppl  and  aal  are  the 
principal  sections  of  the  nicols  and  nop  =  45°  =  n^p,.  On  the  other 
hand,  the  intensity  of  the  light  ring  is  0  at  a,  a^  p,  andj?,.  The  same 
is  true  for  all  the  other  light  rings,  and  the  whole  figure  of  alternating 
concentric  dark  and  light  rings  is  therefore  traversed  by  a  dark  cross. 


UNI  AXIAL  INTERFERENCE  FIGURE.  71 

whose  arms  are  parallel  to  the  principal  sections  of  the  polarizer  and 
analyzer  (Fig.  22).  Since  all  radial  directions  about  the  optic  axis  are 
alike,  a  rotation  of  the  plate  in  its  plane  does  not  alter  the  phenomena, 
the  cross  and  rings  remaining  fixed. 


Fig.  23 


Between  parallel  nicols  the  appearance  is  reversed,  those  parts  winch 

were  dark  before  being  light,  and  the  light  parts  being  dark  (Fig.  23). 

If  white  light  be  used  instead  of  homogeneous  light,  then  in  the 

place  where  a  black  ring  occurs  for  red  light  will  be  a  light  ring  for 

yellow,  green,  and  other  kinds  of  light.     Since  the  color  depends  on 

(0  _  e\  y 

A,  and  the  value  v  -  '-  —  can  only  be  a  minimum  for  one  color,  there- 
A 

fore  with  white  light  between  crossed  nicols  there  will  be  a  series  of 
colored  rings,  or  isochromatic  circles.  The  dark  cross  parallel  to  the 
principal  sections  of  the  nicols  must  also  be  present  for  white  light, 
because  it  is  determined  by  the  factor  sin  2p  sin  2(0  —  p),  which  is  in- 
dependent of  the  wave-length.  With  parallel  nicols  a  white  cross  will 
replace  the  dark  one,  and  the  colors  of  the  isochromatic  circles  will  be 
the  complementary  ones  to  those  which  appear  between  crossed  nicols. 

If  the  observed  plate  is  cut  exactly  parallel  to  the  basal  plane  of 
the  crystal,  the  locus  of  its  optic  axis  will  coincide  exactly  with  the 
optic  axis  of  the  microscope  ;  it  will  lie  at  the  intersection  of  the  cross- 
wires,  which  will  bisect  the  arms  of  the  cross  of  the  interference  figure, 
and  this  will  'not  alter  its  position  upon  the  rotation  of  the  stage  of 
the  microscope.  But  if  the  section  is  not  parallel  to  the  base,  the  op- 
tic axis  of  the  plate  will  be  inclined  to  the  axis  of  the  microscope,  and 
the  interference  figure  will  be  eccentric  in  the  field  of  view.  During 
a  revolution  of  the  section  the  centre  of  the  interference  figure  will 
describe  a  circle  about  the  point  of  intersection  of  the  cross-wires, 
whose  radius  is  proportional  to  the  inclination  of  the  plate  to  its  optic 
axis. 

The  inclination  may  be  so  great  that  only  a  small  peripheral  part 
of  the  interference  figure  can  be  seen  at  one  time.  Such  uniaxial  in- 


72  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

terference  figures  are  distinguished  from  those  of  biaxial  bodies  by  the 
fact  that  for  every  position  during  a  complete  rotation  of  the  plate  the 
arms  of  the  cross  move  parallel  to  themselves  and  to  the  cross-wires. 

Uniaxial  plates  cut  parallel  to  the  axis  observed  in  convergent  po- 
larized light  exhibit  in  homogeneous  light  alternating  dark  and  light 
curves,  and  in  white  light  colored  curves  of  hyperbolic  form  whose 
distinctness  increases  with  the  thinness  of  the  plate.  They  are  of  no 
practical  importance  for  mineral  diagnosis,  but  must  not  be  confounded 
with  isochromatic  curves  of  biaxial  crystals,  as  may  easily  happen  upon 
a  superficial  inspection. 

Plates  of  Biaxial  Crystals  cut  Perpendicular  to  an  Axis  in  Conver- 
gent Polarized  Light. 

Since  in  all  biaxial  crystals  the  optic  axes  have  different 
positions  for  different  colors  (are  dispersed),  strictly  speaking  a 
mineral  plate  can  only  be  normal  to  an  optic  axis  for  light  of~a~psr- 
ticular  wave-length.  But  the  dispersion  of  the  optic  axes  in  most 
cases  is  so  small  that  it  may  be  neglected.  Let  us  assume  that  the 
angle  between  the  optic  axis  is  great  (60°  —  90°),  for  otherwise  the 
phenomena  in  plates  perpendicular  to  an  axis  would  be  identical  with 
those  in  plates  but  slightly  inclined  to  a  bisectrix.  Suppose  the  light 

\c 


employed  is  homogeneous,  and  that  the  nicols  are  crossed.  In  Fig.  24 
let  u  be  the  point  of  emergence  of  the  optic  axis  to  which  the  plate  is 
perpendicular,  ul  the  point  of  emergence  of  the  second  axis,  nnt  the 
projection  of  the  principal  section  of  the  polarizer,  and  at  right  angles 
to  it  that  of  the  principal  section  of  the  analyzer.  Those  ravs  coming 
from  the  polarizer  which  traverse  the  plate  parallel  to  the  "optic  axis 
do  not  alter  their  plane  of  vibration,  and  consequently  are  totally  re- 
flected in  the  analyzer  without  decomposition.  The  plate  is  dark  at 
u.  For  the  same  reason  all  rays  from  the  polarizer  emerging  along 
the  line  nu^  experience  no  decomposition  in  the  plate,  because  its 
principal  plane,  nu  n,u,,  is  parallel  to  their  plane  of  vibration,  conse- 
quently for  them  sin  2  p  =  0.  There  must  be  a  dark  bar  in  the  inter- 


BIAXIAL  INTERFERENCE  FIGURE.  73 

ference  figure  to  which  the  projection  on  it  of  the  plane  of  the  optic 
axes  in  the  plate  is  parallel.  Kays  from  the  polarizer  emerging  at  any 
other  point  of  the  plate  must  be  separated  into  an  ordinary  ray  vibrat- 
ing at  right  angles  to  the  principal  section,  and  an  extraordinary  one 
vibrating  parallel  to  the  principal  section  ;  and,  for  the  same  reason  as 
that  given  for  uniaxial  plates  at  right  angles  to  the  axis,  there  will 
emerge  at  every  point  of  the  plate  two  rays  with  parallel  direction  and 
perpendicular  planes  of  vibration. 

The  principal  section  for  the  rays  emerging  at  a  is  found  by  con- 
necting a  with  u  and  ul9  and,  bisecting  the  angle  uau^  the  extraordi- 
nary ray  emerging  at  a  vibrates  parallel  to  the  line  bisecting  the  angle 
•uaUtf  the  ordinary  ray  vibrates  at  right  angles  to  it.  If  a^u  is  drawn 
parallel  to  the  line  bisecting  the  angle  uau^  then  the  angle  n^a1  —  p. 
Both  of  the  rays  emerging  at  a  will  be  separated  in  the  analyzer  into 
ordinary  and  extraordinary  components ;  the  ordinary  components 
will  be  totally  reflected  on  the  balsam  film,  and  the  extraordinary  com- 
ponents will  produce  an  interference  ray  of  the  intensity 

P  =  sin2  p  sin  2(0  -  p)  sin'  n  (fLZLfLT. 

If  \°~~e)  L  is  a  whole  number,  that  is,  if  one  ray  has  gained  upon  the 
A 

other  in  the  plate  by  a  number  of  whole  wave-lengths,  then  P  '=  0 
and  the  plate  is  dark  at  a.  All  points  for  which  the  difference  of  phase 
of  the  two  rays  is  the  same  number  of  whole  wave-lengths,  as  at  a,  may 
be  united  with  the  point  a  to  form  a  dark  curve.  But  since  in  biax- 
ial crystals  the  differences  of  elasticity  are  not  the  same  for  all  direc- 
tions which  have  the  same  inclination  to  an  optic  axis,  but  are  different 
.and  are  of  such  a  kind  that  the  difference  from  90°  to  90°  reaches  a 
maximum  and  a  minimum,  then  this  dark  curve  cannot  be  a  circle, 
but  must  be  an  ellipse.  The  eccentricity  of  this  ellipse,  however,  is 
very  small,  because  the  three  axes  of  elasticity  differ  but  little  from 
one  another.  If  we  consider  a  point  a'  at  such  a  distance  from  u  in 
the  direction  ua  that  the  difference  of  phase  of  the  two  rays  will  be 

(sy     g\     ~U~  C)yi  I  1 

-^ — T-^—  —  -         — )  then  the  point  a'  will  appear  light,  and   there 
A  2 

(Q e\  y 

must  be  moreover  an  endless  number  of  points  for  which  v — |-f  -  has 

the  same  value,  and  which  therefore  unite  with  a'  to  form  a  light 
elliptical  curve,  concentric  with  the  dark  one  through  a. 


74  PHYSIOGRAPI1Y  OF  THE  ROCK- MAKING  MINERALS. 

Continuing  in  the  same  manner,  it  is  evident  that  plates  which  are 
cut  at  right  angles  to  an  optic  axis  of  an  orthorhombic,  monoclinic,  or 
triclinic  crystal  when  observed  between  crossed  nicols  in  convergent 
polarized  light  must  show  concentric,  light,  and  dark  curves  of  nearly 
circular  form,  which  are  traversed  by  a  dark  bar  parallel  to  the  pro- 
jection of  the  plane  of  the  optic  axes  of  the  plate. 

The  number  of  rings  is  dependent  on  the  difference  o  —  <?,  the 
thickness  of  the  plate,  and  the  wave-length  of  the  light  employed. 
The  form  of  the  interference  figure  is  represented  in  Fig.  25. 

If  white  light  be  used  in  place  of  homogene- 
ous light,  the  dark  bar  will  remain  unchanged, 
since  its  presence  is  independent  of  that  part  of 
the  formula  containing  A,  while  for  the  same 

reasons  that  were  given  in  discussing  the  inter- 
Fig.  fcJQ 

ference  figure  of  a  uniaxial  crystal  there  must 

appear   isochromatic    curves    in    the   place   of    the    dark   and   light 
ones.     That  a  dark  bar  should  appear,  and  not  a  dark  cross  as  in  the 
case  of  uniaxial  interference  curves,  is  due  to  the  fact  that  for  no  other  » 
rays  than  those  emerging  along  the  line  nn1  are  the  principal  sections 
parallel  or  perpendicular  to  that  of  the  analyzer,  as  Fig.  24  shows  for   \ 
the  rays  emerging  at  &,<?,<#. 

If  a  plate  cut  at  right  angles  to  an  axis  of  a  biaxial  mineral  be  ro- 
tated between  crossed  riicols  from  the  position  in  which  it  has  just  been 
discussed,  the  bar  will  bend  into  the  arm  of  a  slightly  curved  hyperbola 
whose  convexity  is  turned  toward  the  second  axis,  and  it  will  straighten 
itself  to  a  bar  again  when  the  plane  of  the  optic  axis  lies  parallel  to  the 
principal  section  of  the  analyzer.  Upon  further  rotation  it  assumes 
the  hyperbolic  curve,  and  after  a  rotation  of  180°  assumes  the  original 
position.  If  the  plate  was  not  cut  exactly  at  right  angles  to  an  optic 
axis,  the  centre  of  the  interference  figure  will  lie  eccentrically  in  the 
field  of  view,  and  during  a  rotation  of  the  plate  in  its  plane  it  will  de- 
scribe a  circle  about  the  point  of  intersection  of  the  cross-wires.  The 
dark  bar  always  bisects  the  field  of  view  when  the  plane  of  the  optic1 
axes  is  parallel  or  at  right  angles  to  the  principal  plane  of  the  polarizer ;. 
in  all  other  positions  it  shows  distinctly  the  form  of  an  hyperbola 
whose  pole  coincides  with  the  locus  of  the  optic  axis  within  the  field 
of  view,  and  whose  convexity  is  turned  toward  the.  pole  of  the  other 
axis. 

Plates  of  biaxial  minerals  cut  perpendicular  to  the  acute  bisectrix, 
examined  in  convergent  polarized  light  between  crossed  nicols  with 
homogeneous  light,  when  the  angle  between  the  optic  axes  is  not  too. 


BIAXIAL  INTERFERENCE  FIGURE.  75 

great,  exhibit  light  and  dark  curves,  which  may  be  easily  derived  from 
what  has  been  said  of  the  origin  of  such  curves  in  plates  cut  perpen- 
dicular to  an  optic  axis.  If  the  plane  of  the  optic  axes  in  the  plate 
lie  parallel  to  the  principal  section  of  the  polarizer  or  analyzer,  then 
the  loci  of  both  the  axes  lying  at  equal  distances  from  the  centre  of  the 
field  of  view  will  be  dark,  and  each  axial  point  will  be  surrounded  by 
closed,  light,  and  dark  curves,  part  of  which  enclose  only  one  axis,  and 
part  both  axes  together,  and  belong  to  a  group  of  lemniscates.  The 
innermost  axial  rings  have  an  oval  form,  consequently  there  is  one 
curve  which  has  the  shape  of  an  oo,  and  whose  point  of  intersection 
lies  in  the  centre  of  the  interference  figure. 

For  very  weak  double  refraction  or  very  slight  thickness  of  the 

t0 e)V 

plate,  thus  for  a  small  value  for- — - — - — ,  and  consequent  great  dis- 

A 

(0 e\  Y 

tance  apart  of  points  for  which — —  =  n,  the  inner,  separated 

curves  are  wanting  from  about  the  axial  points  and  toe  first  dark  lem- 
niscate  encloses  both  axes. 

The  interference  figure  in  the  position  given  is  traversed  by  a 
small  and  sharp  dark  bar  which  connects  the  two  axial  points,  and  also 
by  a  broad  dark  stripe  increasing  rapidly  in  width  outward  which 
stands  perpendicular  to  this  direction  (Fig.  26).  Both  dark  bars  arise 
from  the  fact  that  the  rays  from  the  polarizer  have  not  suffered  any 


Fig.  26  Fig.  3?" 

decomposition  in  the  plate  along  these  lines  because  their  plane  of 
vibration  stands  parallel  or  perpendicular  to  the  principal  section  of 
the  plate  ;  they  enter  the  analyzer  as  ordinary  rays  and  become  totally 
reflected  from  the  balsam  film. 

If  the  observation  is  made  with  white  light,  the  lemniscates  instead 
of  being  light  and  dark  will  be  variously  colored,  and  isochromatic. 
The  dark  bars,  as  before,  will  remain  unchanged. 

If  the  plate  is  rotated  between  crossed  nicols  from  the  position  in 


76 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


which  the  plane  of  the  optic  axes  is  parallel  to  the  principal  section  01 
the  polarizer  or  analyzer  (parallel  position),  the  isochromatic  curves 
about  the  axes  remain  unchanged  except  that  they  rotate  with  the  axes 
in  the  field.  But  the  dark  cross  opens  to  equilateral  hyperbolas  whose 
poles  each  lie  at  the  locus  of  an  optic  axis,  and  after  a  rotation  of  45° 
(diagonal  position)  the  interference  figure  has  the  appearance  shown 
in  Fig.  27. 

If  the  principal  sections  of  the  nicols  are  parallel,  the  complement- 
ary phenomena  are  presented,  as  has  been  explained  for  plates  of  uni- 
axial  substances.  The  distance  apart  of  the  loci  of  the  optic  axes  is 
naturally  a  measure  of  the  size  of  the  angle  between  the  optic  axes, 
and  is  therefore  independent  of  the  thickness  of  the  plate. 

Dispersion  of  the  Axes. — The  interference  figures  of  biaxial 
plates  cut  perpendicular  to  the  acute  bisectrix  serve  to  show  the  dis- 
persions of  the  optical  constants,  which  are  different  for  each  of  the 
crystallographic  systems  without  a  principal  axis,  and  therefore  furnish 
conclusive  evidence  as  to  the  crystal  system  of  the  plate  under  investi- 
gation. It  can  be  taken  as  a  rule  that  the  grade  of  symmetry  of  an 
interference  figure  is  the  same  as  that  of  the  crystal  face  on  which  it 
lies,  and  that  the  same  planes  are  planes  of  symmetry  for  the  crystal 
faces  and  for  the  interference  figure. 

In  the  orthorhombic  system  the  bisectrices  cannot  experience  any 
dispersion,  but  the  optic  axes  may,  under  condition  that  their  disper- 
sion, p  >  v  or  p  <  v,  must  be  symmetrical  with  respect  to  the  bisec- 


trices. Therefore  if  a  plate  cut  perpendicular  to  the  bisectrix  of  an 
orthorhombic  mineral  be  observed  between  crossed  nicols  in  red  and 
then  in  blue  light,  the  loci  of  the  axes  and  also  the  dark  rings  will  not 
coincide  in  the  two  cases,  but  according  as  the  angle  of  the  red  or  of 
the  blue  axes  is  greater  they  will  have  different  positions.  In  Fig.  28 
it  is  assumed  that  the  angle  of  the  red  axes  is  the  smaller,  and  that  at  p 


DISPERSION  IN  ORT110RHOMBIC  CRYSTALS. 


77 


will  lie  the  dark  axial  spots  and  the  poles  of  the  hyperbolas  in  the  diag- 
onal position,  and  pl  will  be  the  first  dark  ring  in  red  light.  In  blue 
light  the  poles  of  the  dark  hyperbolas  may  be  assumed  to  lie  at  v,  and 
the  first  ring  at  v^  Between  p,  and  vl  will  lie  the  first  dark  rings,  and 
between  p  and  v  the  dark  hyperbolas,  for  orange,  yellow,  and  green. 
Then  in  white  light  there  will  appear  on  the  first  ring  at  pl  a  combina- 
tion color  in  which  red  is  excluded ;  at  vl  one  in  which  blue  is  excluded. 
This  first  colored  ring,  therefore,  will  appear  red  at  the  spot  lying  near- 
est the  middle  point  or  within,  that  i£,  nearest  the  pole  of  the  hyper- 
bola ;  the  second,  which  is  without  or  farthest  from  the  pole,  will  be 
blue.  The  colors  in  the  outer  rings  blend  into  one  another  because  so 
many  light  rings  are  superimposed,  but  within  they  are  more  distinct. 
The  inner  red,  therefore,  is  very  distinctly  seen,  while  the  outer  blue 
is  less  so.  In  that  part  of  the  first  ring  which  is  farthest  from  the 
centre  of  the  interference  figure  the  order  is  naturally  reversed,  blue 
being  within  and  red  without.  The  first  is  distinct,  the  second  indis- 
tinct or  blended.  It  is  the  same  with  the  other  rings,  but  only  the 
innermost  ring  is  used  for  observation  because  the  phenomenon  in  it  is 
clearest. 

On  the  dark  hyperbolas  which  occur  between  v  and  p,  Fig.  28,  red 
is  extinguished  on  the  convex  side,  and  this  must  therefore  appear 
edged  with  blue.  On  the  other  hand,  blue  is  extinguished  on  the  con- 
cave side,  and  this  must  be  edged  with  red.  From  this  is  derived  tha 


v ? i 


Fig. 


SO 


rule  that  the  axial  angle  is  smallest  for  the  color  which  appears  within 
that  part  of  the  first  ring  which  is  nearest  the  centre  of  the  figure,  and 
which  in  the  diagonal  position  borders  the  concave  side  of  the  hyper- 
bola. On  the  other  hand,  that  color  appears  on  the  convex  side  of  the 
hyperbola  and  in  that  part  of  the  innermost  ring  farthest  from  the 
centre  of  the  figure  for  which  the  axial  angle  is  the  greatest.  Figs.  29 


78 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


and  30  show  orthorhombic  interference  figures  in  parallel  and  diagonal 
position  with  the  dispersion  p<v  (the  red  being  indicated  by  stippling, 
the  blue  by  lining).  They  show  as  in  Fig.  28  that  the  figures  are  sym- 
metrical with  respect  to  the  axial  plane  AA^  and  to  one,  BB^  normal 
to  it.  The  interference  figure  lies  on  a  pinacoid  of  the  orthorhombic 
system,  and  like  this  is  therefore  bisymmetric. 

In  the  monodinic  system  the  relative  size  of  the  axial  angle  may 
be  recognized  by  the  same  phenomena  as  in  the  orthorhombic  interfer- 
ence figures.  But  there  occurs  in  this  a  particular  disposition  of  the 
colors  which  is  determined  by  the  dispersion  of  the  axes  of  elasticity 

lying  in  the  plane  of  symmetry,  and  which 
is  added  to  the  dispersion  of  the  optic  axes. 
If  the  orthodiagonal  is  the  axis  of  mean 
elasticity  and  the  plane  of  the  optic  axis, 
therefore,  lies  in  the  plane  of  symmetry, 
then  both  bisectrices  for  the  different  colors  are  dispersed  and  one  can- 
not properly  speak  of  a  section  at  right  angles  to  the  acute  bisectrix, 
but  only  of  one  perpendicular  to  one  of  the  acute  bisectrices.  For  this 
dispersion  (the  inclined)  the  dark  axial  spots  between  crossed  nicols  in 
red  light  would  appear  at  p  (Fig.  31)  equally  distant  from  the  locus  of 


JTig.  31 


Fig.  33 


the  bisectrix  Bft ,  and  the  first  dark  rings  in  place  of  the  small  circles 
about  p.  For  the  case/x;^  the  dark  axial  spots  in  blue  light  would  lie 
at  v  equally  distant  from  Bv,  and  the  first  dark  rings  in  place  of  the 
larger  circles  about  v.  The  axial  figures  are  thus  displaced  with 


DISPERSION  IX  MOXOCLIXIC  CRYSTALS. 


79 


respect  to  one  another,  and  on  using  white  light  the  disposition  of  the 
colors  within  the  inner  rings  and  on  the  edges  of  the  hyperbolas  is 
symmetrical  to  the  plane  of  the  bisectrices,  but  not  to  one  at  right 
angles  to  it.  If  the  dispersion  of  the  bisectrices  is  not  large,  then  the 
colors  lie  symmetrical  with  respect  to  the  dark  bar  at  right  angles  to 
the  plane  of  the  optic  axes  as  their  order  of  succession  goes,  but  they 
have  different  intensities  on  either  side,  and  the  inner  rings  are  more 
elliptical  on  one  side  than  on  the  other,  as  is  shown  in  Figs.  32  and  33, 
which  have  been  derived  from  the  scheme  for  gypsum,  Fig.  31.  With 
more  strongly  inclined  dispersion,  as  in  diopside,  the  inner  rings  in 
analogous  places  and  the  hyperbolas  on  the  same  sides  are  colored  dif- 
ferently ;  that  is,  on  one  side  they  are  red  within  and  blue  without,  and 
•on  the  other  blue  within  and  red  without,  as  shown  in  Figs.  34  and  35. 
In  other  cases  both  of  these  forms  of  appearance  are  combined. 

For  horizontal  dispersion,  that  is,  when  b  is  the  obtuse  bisectrix 


with  the  normal  symmetrical  position  of  the  optic  axes,  the  axial  fig- 
ures with  crossed  nicols  for  red  and  blue  color  will  have  the  position 
with  respect  to  one  another  indicated  in  Fig.  36,  and  there  can  be  no 
section  which  will  be  at  right  angles  to  all  acute  bisectrices  at  once. 


38  Fig.  39 

The  obtuse  bisectrix  is  the  same  for  all  colors  ;  since  it  coincides  with 
the  axis  of  symmetry,  the  acute  bisectrices  and  the  axes  of  mean  elas- 
ticity are  dispersed.  In  white  light  the  superposition  of  the  axial  fig- 
ures for  the  different  wave-lengths  must  produce  a  disposition  of  the 
colors  which,  when  the  dispersion  is  p  >  -y,  is  shown  in  Fig.  37  for 
the  parallel  position,  and  in  Fig.  38  for  the  diagonal  position.  The 


80  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

disposition  of  the  colors  is  now  symmetrical  with  respect  to  a  plane 
normal  to  ail  of  the  axial  planes,  for  the  different  colors,  which  corre- 
sponds to  the  crystallographic  plane  of  symmetry.  Finally,  if  the  axis 
of  symmetry  I  is  the  acute  bisectrix,  the  dispersion  is  crossed,  for  the 
axial  planes  for  red  and  blue  light  will  show  a  displacement  with  re- 
spect to  one  another,  as  shown  in  Fig.  39.  The  acute  bisectrix  is  then 
the  same  for  all  colors ;  the  obtuse  bisectrix  and  axis  of  mean  elasticity 
are  dispersed.  In  white  light  the  partially  overlapping  axial  figures 
would  produce  a  disposition  of  colors,  for  p  >  v,  which  is  shown  in 
Fig.  40  for  the  parallel  position,  and  in  Fig.  41  for  the  diagonal 


40  Fig.  4.1 


tion.  There  is  no  longer  any  plane  with  respect  to  which  the  inter- 
ference figure  is  symmetrical,  for  it  lies  within  the  plane  of  symmetry. 
But  the  disposition  of  the  colors  is  symmetrical  with  respect  to  the 
locus  of  the  acute  bisectrix,  since  this  is  the  crystallographical  axis  of 
sym  metry . 

In  triclinic  crystals  all  the  axes  of  elasticity  are  dispersed  ;  the  dis- 
persions shown  by  the  interference  figure  in  convergent  light  are 
therefore  different  for  different  colors.  The  disposition  of  the  colors 
in  the  axial  figures  is  consequently  entirely  unsym metrical. 

Determination    of   the    Optical    Character    of  Doubly   Refracting 
Plates  in  Convergent  Light. 

The  commonest  methods  for  determining  the  optical  character  of 
doubly  refracting  minerals  in  convergent  light  are  based  on  the  fact 
that  the  isochromatic  curves  of  an  interference  figure,  since  they 
unite  all  the  points  of  a  plate  at  which  the  emerging  rays  suffer  the 
same  retardation,  must  experience  an  alteration  if  the  difference  of 
phase  of  the  rays  is  increased  or  diminished.  With  uniaxial  crystals 
this  is  accomplished  most  conveniently  by  the  insertion  of  a  mica 
plate  of  such  thickness  that  the  two  component  rays  produced  in  it  by 


DETERMINATION  OF  OPTICAL  CHARACTER. 


81 


double  refraction  emerge  with  a  phasal  difference  of  J  of  a  wave- 
length (Viertelundulationsglimmerblattchen).  Since  mica  is  optically 
negative,  and  the  plane  of  the  optic  axes  stands  very  nearly  perpendic- 
ular to  the  cleavage  plane,  this  latter  plane  contains  the  axes  of  smallest 
and  of  mean  elasticity ;  the  former  coincides  with  the  line  connecting 
the  loci  of  the  optic  axes,  and  the  latter  is  at  right  angles  to  it.  The 
direction  of  the  plane  of  the  optic  axes,  and  consequently  of  the  axis 
C,  is  marked  on  the  mica  plate  or  on  the  frame  holding  it.  If  one  ob- 
serves the  interference  figure  of  a  uniaxial  crystal  after  the  mica  plate 
has  been  so  placed  between  the  object  and  the  analyzer  that  its  axial 
plane  is  inclined  45°  to  the  principal  sections  of  the  crossed  nicols, 
the  dark  cross  will  have  resolved  itself  into  two  dark  spots,  d  and  <#„ 
in  Figs.  42  and  43,  and  the  isochromatic  curves  in  two  opposite  quad- 


rants will  have  contracted,  and  in  the  other  two  will  have  expanded. 
The  crystal  under  investigation  is  optically  positive  (Fig.  42)  when  the 
line  joining  the  dark  spots  is  at  right  angles  to  the  axial  plane  of  the 
mica  plate  (forms  with  it  the  sign  -f-) ;  the  crystal,  on  the  other  hand, 
is  negative  (Fig  43)  when  this  connecting  line  is  parallel  to  the  axial 
plane  of  the  mica  plate  (— ).  This  is  explained  by  the  following  con- 
sideration :  Let  PPl  and  A A}  (Fig.  42)  be  the  principal  sections  of 
the  polarizer  and  analyzer,  gy^  the  axial  plane  of  the  mica  plate,  and  in 
homogeneous  light  let  aba  be  the  first  dark  ring,  that  is,  the  location 


of  all  points  of  the  plate  for  which 


(o-e)V 


—  1.     The  plate  is  as- 


sumed to  be  optically  positive,  C  =  a.  Of  the  rays  emerging  at  the 
point  a  of  the  first  ring,  the  extraordinary  will  vibrate  in  the  principal 
section  ac,  and  will  be  retarded  a  wave-length  behind  the  ray  vibrating 
at  right  angles  to  the  principal  section.  In  passing  through  the  mica 


82  PHYSIOGRAPHY   OF  THE  ROCK-MAKING  MINERALS. 

plate  the  extraordinary  ray  will  be  again  retarded  £A  behind  the  ordi- 
nary ray,  since  it  vibrates  parallel  to  the  axis  of  least  elasticity  in  the 
mica.  The  phasal  difference  of  both  rays  when  they  enter  the  analy- 
zer is  therefore  fA  and  the  point  a  can  no  longer  be  dark  ;  the  dark 
ring  has  therefore, moved,  and  will  be  found  at  a  point  al  where  the 
phasal  difference  of  the  rays  emerging  from  the  plate  is  f  A,  which  will 
become  A  on  passing  through  the  mica  plate.  The  same  takes  place  in 
the  opposite  quadrant  PcAr 

For  the  rays  emerging  at  the  point  1)  of  the  first  dark  ring  the 
principal  section  IB 'be.  Here  also  the  extraordinary  ray  is  retarded  a 
wave-length  behind  the  ordinary.  But  upon  their  entrance  in  the 
mica  plate  the  extraordinary  ray  vibrates  parallel  to  the  axis  of  greater 
elasticity.  The  extraordinary  ray  is  therefore  accelerated  with  respect 
to  the  ordinary  one,  and  by  J  of  a  wave-length.  Upon  entering  the 
analyzer  the  phasal  difference  will  be  only  f  A  and  this  spot  will  not 
appear  dark.  The  first  dark  ring  will  be  moved  to  a  point  b^  where 
the  phasal  difference  of  the  rays  emerging  from  the  plate  is  f  A.  The 
first  dark  ring  is  thus  divided  into  four  arcs  of  90°.  In  those  quadrants 
through  which  the  axial  plane  (gg^)  of  the  mica  plate  passes  there 
occurs  a  contraction,  and  in  the  other  two  quadrants  a  dilation.  The 
same  change  affects  the  other  rings  of  the  figure. 

The  reverse  phenomenon  of  Fig.  43,  which  is  found  for  negative 
uniaxial  crystals,  is  explained  in  a  similar  manner. 

In  plates  of  biaxial  crystals,  also,  cut  perpendicular  to  the  bisectrix 
the  character  of  the  double  refraction  may  be  determined  by  means  of 
the  mica  plate.  The  plate  for  investigation  is  placed  in  parallel  posi- 
tion between  crossed  nicols,  and  the  mica  plate  is  inserted  so  that  its 
axial  plane  bisects  the  angle  between  the  principal  sections  of  the 
nicols.  If  the  plate  in  question  is  optically  positive,  its  bisectrix 
being  the  axis  of  least  elasticity,  then  the  axial  rings  are  contracted  in 
the  quadrants  through  which  the  axial  plane  of  the  mica  plate  passes, 
and  are  widened  in  both  the  others. 

With  negative  character  of  the  plate  under  investigation  the  axial 
rings  are  dilated  in  the  quadrants  through  which  the  axial  plane  of  the 
mica  plate  passes,  and  contracted  in  both  the  others. 

The  phenomenon  of  the  dilation  and  contraction  of  the  axial  rings 
effected  by  the  mica  plate  is  not  always  easily  recognized.  It  is  then 
better  to  employ  a  quartz  wedge  such  as  already  described  for  deter- 
mining the.  optical  character  of  doubly  refracting  plates  through  the 
change  in  their  interference  color.  In  such  a  quartz  wedge,  since 
quartz  is  optically  positive  the  axis  of  greater  elasticity  lies  parallel  to 


COLOR  OF  MINERALS.  83 

the  thin  edge  of  the  wedge  ;  the  axis  of  least  elasticity  lies  parallel  to 
the  Ions'  ed^e. 

O  C5 

If  a  plate  cut  perpendicular  to  the  bisectrix  is  placed  in  the 
diagonal  position  between  crossed  nicols,  there  will  be  a  certain  number 
of  axial  rings  in  the  interference  figure  which  surround  each  axis 
separately,  and  around  both  of  these  groups  will  be  the  lemniscates. 
For  optically  positive  crystals  the  bisectrix  perpendicular  to  the  plate 
is  c ;  the  line  connecting  the  hyperbolas  =  a,  that  at  right  angles  to 
this  =  b.  If  the  quartz  wedge  is  inserted  horizontally  anywhere 
between  the  plate  and  the  analyzer,  so  that  .the  long  edge  (c)  will  ad-| 
vance  parallel  to  the  line  connecting  the  hyperbolas,  then  the  ray 
which  in  the  plate  vibrates  parallel  to  a  and  is  accelerated  will  vibrate 
parallel  to  C  in  the  quartz  wedge  and  will  be  retarded.  The  ray 
vibrating  parallel  to  b  in  the  plate  and  there  retarded  will  in  the 
quartz  wedge  vibrate  parallel  to  a  and  be  accelerated  ;  therefore  the 
two  rays  emerging  at  any  point  of  the  first  dark  circle  whose  phasal 
difference  in  the  plate  is  A,  will  have  a  smaller  difference  ;  the  same  is 
true  of  all  points  of  the  second  dark  ring,  and  so  on  :  the  rings  must 
therefore  widen.  The  farther  forward  the  quartz  wedge  is  pushed 
the  wider  the  rings  must  become.  They  move  from  the  axial  spots 
toward  the  centre  of  the  interference  figure,  and  'finally  open  into  lem- 
niscates. It  is  as  though  the  plate  became  thinner  and  thinner. 

If  the  long  edge  of  the  quartz  wedge  be  moved  at  right  angles  to 
the  line  connecting  the  hyperbolas,  then  a  in  the  quartz  coincides  with 
a  in  the  plate,  and  C  in  the  quartz  coincides  with  c  in  the  plate.  The 
effect  is  as  though,  the  thickness  of  the  plate  were  increased,  and  with 
the  advance  of  the  quartz  wedge  the  axial  rings  will  approach  the 
axial  spot.  The  movement  of  the  interference  figure  will  be  found 
to  be  from  without  inward. 

For  negative  crystals  the  phenomena  are  reversed. 

e.   Color  of 'Minerals. 

The  color  of  minerals  in  reflected  light  can  only  be  used  as  a  crite- 
rion when  it  is  an  inherent  color,  and  especially  when  the  minerals  will 
not  yield  transparent  plates,  which  is  particularly  the  case  with  the  ores, 
many  of  which  in  the  series  of  oxide  ores,  as  magnetite,  ilmenite,  and 
hematite,  are  among  the  most  widely  distributed  constituents  of  rocks, 
while  others  in  the  group  of  the  sulphide  ores,  as  pyriteand  pyrrhotite, 
are  frequently  accessory  constituents.  To  the  idiochromatic  trans- 
parent minerals  which  are  widely  distributed  in  rocks  belong  certain 


84  PHYSIOGRAPHY  OF  THE  ROCK-MAKING   MINERALS. 

oxides,  as  rutile,  and  especially  silicates  with  heavy  metallic  bases,  as  the 
micas,  pyroxenes,  amphiboles,  garnets,  tourmaline,  etc.  Color  can 
seldom  be  used  for  determining  minerals  in  consequence  of  the  great 
variety  of  the  colors  which  are  due  to  the  stage  of  oxidation  of  the 
metals  (iron  and  manganese),  and  to  their  relative  proportions  in  combi- 
nation with  isomorphons  molecules  of  other  elements.  In  the  case  of 
stained  minerals,  which  are  of  themselves  colorless  and  only  derive 
their  color  from  foreign  substances  of  inorganic  nature,  the  color  has 
naturally  no  determinative  importance.  The  microscopical  investigation 
of  such  allochromatic  substances  shows  either  that  their  pigment  is 
present  in  well-defined  and  recognizable  minute  plates,  needles,  or 
grains,  in  the  form  of  inclusions,  and  then  very  often  irregularly  dis- 
seminated through  the  whole  mass,  or  that  the  coloring  matter  can- 
not be  recognized  as  separate  from  the  colored  mass.  When  a  pig- 
ment is  present  in  the  latter  form  it  is  called  a  dilute  one,  and  it  is  a 
peculiarity  of  such  dilutely  stained  bodies  that  their  color  disappears- 
more  or  less  completely  when  they  are  cut  sufficiently  thin.  It  is  gen- 
erally assumed — and  the  phenomenon  of  pleochroism  in  allochromatic,, 
dilutely  stained  bodies  necessitates  this  assumption — that  in  this  case 
the  pigment  is  dispersed  through  the  intermolecular  spaces  of  the  sub- 
stance. Chemical  investigation  has  shown  that  extremely  small 
amounts  of  dilute  pigments  can  often  produce  a  very  intense  colora- 
tion. 

If  the  color  of  a  body  in  incident  light  arises  from  the  fact  that  not 
all  of  the  rays  of  incident  white  light  but  only  those  of  certain  wave- 
lengths are  reflected,  while  those  of  other  wave-lengths  are  absorbed, 
then  the  colors  of  this  body  in  transmitted  light  are  determined  by  the 
absorption  of  certain  rays.  It  is  known  that  luminous  waves  are 
always  weakened  by  their  passage  through  transparent  media;  that  they 
are  indeed  completely  extinguished  when  the  thickness  of  the  layer 
traversed  is  sufficiently  great.  Now,  since  the  elasticity  of  the  lurninif- 
erous  ether  in  isotropic  media  is  the  same  in  all  directions,  it  must  be 
assumed  that  the  weakening  of  a  luminous  wave  traversing  them  will 
be  independent  of  its  direction.  Their  degree  of  transparency  there- 
fore must  only  depend  on  a  coefficient  of  absorption  peculiar  to  the 
substance,  and  on  its  thickness,  if  the  rays  of  all  wave-lengths  are 
equally  absorbed.  If  the  absorption  is  especially  confined  to  rays  of 
certain  wave-lengths,  its  color  must  still  be  independent  of  the  direc- 
tion. 

It  is  different  with  anisotropic  media,  since  in  them  the  elasticity 
of  the  ether  and  therefore  the  velocity  of  the  light  changes  with  the 


PLEOCHROISM.  85 

direction,  and  it  may  be  assumed  that  the  absorption  of  the  light 
would  also  be  different  in  different  directions,  and  that  for  like  ab- 
sorption of  rays  of  all  wave-lengths  the  transparency  of  equally  thick 
plates  may  be  different  if  the  plates  have  been  cut  in  different  direc- 
tions. And  if  the  absorption  of  the  rays  for  all  wave-lengths  is  not 
the  same,  rays  of  different  wave-lengths  may  be  absorbed  more  in  one 
direction  than  in  another.  Then  plates  of  such  isotropic  media  which 
have  been  cut  in  different  directions  will  show  different  colors  in 
transmitted  light.  This  phenomenon  of  color-absorption  of  doubly 
refracting  bodies,  wrhich  changes  with  the  direction,  is  called  pleochro- 
ism. 

Pleochroism  of  Uniaxial  Minerals. — If  a  uniaxial  crystal  is  looked 
at  in  such  a  way  that  the  rays  of  light  strike  at  right  angles  to 
its  base,  only  ordinary  rays  will  reach  the  eye,  and  the  color  shown  by 
the  crystal  in  this  direction  (basal  color)  is  determined  exclusively  by 
the  absorption  which  the  ordinary  ray  experiences.  If  these  rays  are 
investigated  in  any  way  by  a  uicol,  the  color  will  always  remain  the 
same  in  whatever  way  the  nicol  may  be  turned  about  its  axis.  There 
is  no  double  refraction  in  the  direction  of  the  principal  axis.  If  now 
the  crystal  (tourmaline,  beryl,  vesuvianite,  etc.)  be  viewed  in  a  direc- 
tion inclined  to  the  principal  axis,  the  color  will  be  changed,  and  will 
differ  more  from  the  basal  color  as  the  inclination  of  the  ray  to  the 
principal  axis  is  greater.  The  maximum  difference  in  color  must  be 
seen  when  the  crystal  is  viewed  at  right  angles  to  the  principal  axis. 
This  phenomenon  is  explained  by  the  fact  that  for  an  inclined  position 
of  the  principal  axis  to  the  direction  of  the  rays  a  double  refraction  of 
the  rays  takes  place ;  with  the  ordinary  ray  is  associated  an  extraor- 
dinary ray  whose  velocity  and  absorption  differ  the  more  from  those 
of  the  ordinary  ray  the  nearer  the  principal  axis  lies  to  the  direction 
of  the  rays.  Consequently  the  colors  shown  by  a  uniaxial  crystal  in 
any  other  direction  than  that  of  its  principal  axis  are  determined  by 
the  combination  of  the  ordinary  and  extraordinary  rays,  each  of  which 
is  absorbed  differently. 

It  is  customary  to  consider  only  the  extreme  cases,  that  is,  when 
the  principal  axis  is  parallel  and  perpendicular  to  the  direction  of  the 
rays,  and  to  say  that  uniaxial  crystals  possess  dichroism;  which  is  not 
strictly  correct,  since  the  color  changes  steadily  with  the  direction. 

The  absorption  belonging  to  each  of  the  rays  traversing  a  doubly 
refracting  plate  may  be  observed  by  means  of  the  polarizing  micro- 
scope. If  the  polarizer  be  in  place  and  the  analyzer  be  removed,  then, 
by  rotating  the  stage  of  the  microscope  until  the  principal  section  of 


86  PHYSIOGRAPHY  OF  TEE  ROCK-MAKING  MINERALS. 

the  plate  be  brought  first  parallel  and  then  at  right  angles  to  the 
principal  section  of  the  polarizer,  the  plate  in  the  first  case  will  be 
traversed  by  an  extraordinary  ray  only,  and  in  the  other  by  an  ordinary 
ray,  and  the  particular  absorption  of  each  can  be  tested.  If  the  polar- 
izer be  removed  and  the  analyzer  retained,  it  will  be  found  that  a  por- 
tion of  the  light  reflected  from  the  mirror  is  polarized,  which  interferes 
with  the  observation. 

Pleochroisin  of  Biaxial  Minerals. — Plates  of  biaxial  minerals  cut 
perpendicular  to  an  optic  axis  usually  show  no  pleochroism,  but  all 
other  plates  of  such  minerals  show  a  color  which  changes  with  the 
direction  of  the  plate,  if  there  is  any  appreciable  color-absorption  pres- 
ent. This  color  is  composed  of  the  colors  of  both  of  the  rays  which 
traverse  the  plata.  If  with  Haidinger  we  call  these  facial  colors 
(Fliichenfarben),  then,  as  in  the  case  of  uniaxial  plates  which  are  not 
cut  perpendicular  to  the  principal  axis,  these  colors  may  be  separated 
by  a  nicol  prism  into  axial  colors,  that  is,  into  the  colors  of  the  indi- 
vidual rays  which  traverse  the  plate.  For  example,  let  Fig.  44  repre- 
sent a  cube  cut  from  an  orthorhombic  crystal  in  such 
a  manner  that  each  face  is  perpendicular  to  an  axis 
of  elasticity;  then  if  \ve  observe  perpendicularly  in- 
cident light  through  the  faces  A,  B,  and  C,  we  shall 
have  three  facial  colors  with  a  maximum  difference  be- 
tween them.  The  facial  color  (7  is  composed  of  rays 
.  44  vibrating  parallel  to  a  and  fc.  In  the  same  way  the 

facial  colors  A  and  B  are  composed  of  rays  vibrating 
parallel  to  b  and  c,  and  a  and  c,  respectively;  therefore  three  facial 
colors  and  three  axial  colors  are  distinguished  in  biaxial  crystals. 

It  was  formerly  assumed  that  the  directions  of  strongest  color- 
absorption  in  biaxial  crystals,  which  may  be  termed  the  axes  of  absorp- 
tion, were  coincident  with  the  axes  of  elasticity.  IL  Laspeyres  has 
shown  that  this  is  only  true  so  long  as  the  axes  of  elasticity  coincide 
with  the  crystallographic  axes  of  symmetry;  consequently  it  is  true  for 
all  three  axes  of  elasticity  in  the  orthorhombic  system,  and  in  the 
monoclinic  system  for  the  axis  coinciding  with  the  axis  of  symmetry, 
b ;  on  the  other  hand,  it  is  not  necessarily  true  for  the  two  axes  of  elas- 
ticity lying  in  the  plane  of  symmetry  of  monoclinic  crystals,  nor  for  all 
the  axes  of  elasticity  of  triclinic  crystals.  Indeed  li.  Laspeyres  found 
that  in  manganese-epidote  a  dispersion  of  the  axes  of  absorption  takes 
place  in  the  plane  of  symmetry  independent  of  the  dispersion  of  the 
axes  of  elasticity  a  and  c. 

Pleochroic  Ilalos. — Many  minerals,  as  andalusite,  cordierite,  mus- 


PLEOCHROIG  UALOS.  87 

covite,  biotite,  diopside,  etc.,  sliow  a  peculiar  phenomenon,  namely, 
that  particular  spots  in  them  possess  a  marked  pleochroism,  especially  in 
the  immediate  vicinity  of  microscopic  inclusions,  so  that,  in  one  of  the 
positions  of  extinction,  after  the  analyzer  has  been  removed  there  are 
strongly  colored  halos  around  the  microscopic  inclusions,  which  halos 
disappear  more  or  less  completely  after  a  rotation  of  the  plate  through 
90°.  In  eordierite*  and  andalusite  these  halos  are  bright  yellow,  and 
arise  from  a  local  aggregation  of  an  organic  pigment ;  they  disappear 
after  the  mineral  has  been  heated  to  redness.  A.  Michel-Levy  f  and 
H.  Gyllingt  found  that  very  high  heating*did  not  destroy  these  halos 
in  certain  micas,  and  the  former  concluded  that  in  this  case  the  phenom- 
enon did  not  arise  from  an  organic  pigment,  but  might  be  occasioned 
by  a  greater  local  percentage  of  ferruginous  molecules.  Against  this 
explanation  is  the  fact  that  the  phenomenon  is  confined  to  the  imme- 
diate vicinity  of  an  inclusion,  and  also  that  the  halos  are  always  oval, 
while  one  would  expect  a  crystallographic  boundary  if  the  phenom- 
enon were  confined  to  an  isomorphous  shell.  Moreover  it  is  well 
known  that  micaceous  and  fibrous  substances  lose  water  with  great 
difficulty,  even  upon  very  strong  and  continued  heating  at  redness.  § 

Colorless  minerals  may  sometimes  be  rendered  pleochroic  by  arti- 
ficial coloration.  Boricky  observed  that  many  minerals  occurring  in 
rocks  (olivine,  bronzite,  cordierite)  which  in  their  natural  state  show 
no  pleochroism,  or  only  a  weak  one,  become  distinctly  pleochroic,  and 
even  strongly  so,  upon  being  heated  to  redness.  He  obtained  the  best 
results  when  the  substance  (in  thin  section)  was  exposed  on  platinum 
foil  to  a  bright  red  heat  for  1.5  to  2  minutes.  Not  infrequently  the 
olivine  in  rocks  of  the  melaphyre  and  basalt  series  is  colored  red  by  a 
more  or  less  advanced  separation  of  Fe2O3.  This  is  accompanied 
quite  often,  but  not  always,  by  distinct  pleochroism. 

*  H.  Eosenbusch,  Die  Steiger  Scbiefer  und  ihre  Contactzone  an  den  Granititen 
von  Barr-Andlau  und  Hohwald.  Strasslmrg,  1877.  p.  221. 

f  Sur  les  noyaux  a  polychroisme  intense  du  mica  noir.     C.  R.  1882.  xciv.  1196. 

\  Nagra  ord  om  Rutil  och  Zirkon  med  sarskild  hansyn  till  deras  sammanvaxning 
med  Glimmer.  G.  F.  F.  1882.  vi.  167. 

§  E.  Cohen  has  recently  demonstrated  that  the  ploochroic  halos  in  the  biotite  of 
certain  granite-porphyries  and  gneisses  are  also  due  to  organic  pigments,  but  that  it 
requires  a  higher  temperature  to  destroy  them  than  is  necessary  to  dissipate  those  in 
muscovite  and  cordierite.  (N.  J.  B.  1888.  B.  I.  165.) 


88  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


Aggregates. 
Literature. 

E.  BERTRAND,  Du  type  crystallin  auquel  on  doit  rapporter  le  Rhabdophane,  d'apres 
les  proprietes  optiques  que  presentent  les  corps  crystallises  affectant  la  forme 
spherolithique.  Bull.  Soc.  min.  Fr.  1880.  III.  58-62  and  1881.  IV.  60-61. 

—  De  1'application  du  microscope  a  1'etude  de  la  mineralogie.  ibidem  1880.   III. 

93-96. 

—  Sur  les  proprietes  optiques  des  corps  crystallises  presentant  la  forme  sphero- 

lithique.    C.  R.  1882.  XCIV.  542. 

D.  BREWSTER,  On  circular  crystals.     Trans.  Roy.  Soc.  1853.  XX.  part  4.  607-623. 

E.  MALLARD,   Sur  quelques  phenomenes  de  polarisation  chromatique.      Bull.  Soc. 

min.  Fr.  1881.  IV.  66-71. 

A.  MICHEL-LEVY,  Des  differentes  formes  de  spherolithes  dans  les  roches  eruptives 
in:  Memoire  sur  la  variolite  de  la  Durance.  Bull.  Soc.  geol.  Fr.  (3).  V. 
257-266. 

—  Sur  la  nature  des  spherolithes  faisant  partie  integrate  des  roches  eruptives.     C.  R. 
1882.  XCIV.  465. 

H.  ROSENBUSCH,  Einige  Mittheilungen  liber  Zusamniensetzung  und  Structur  grani- 
tischer  Gesteine.     Z.  D.  G.  G.  1876.  XXVIII.  369-390. 

For  reasons  already  given  this  subject  has  been  transferred  from 
the  chapter  on  the  morphological  characters  of  rock-making  minerals 
to  this  place.  The  term  aggregates,  as  here  used,  includes  only  those 
aggregations  which  are  homogeneous  or  cannot  be  shown  to  be  heter- 
ogeneous. They  may  consist  of  amorphous  or  of  crystalline  sub- 
stances. In  general  the  texture  of  amorphous  aggregates  can  only  be 
detected  microscopically  when  they  have  been  rendered  doubly  refract- 
ing from  mechanical  causes.  They  then  behave  like  those  crystalline 
aggregates  which  do  not  belong  to  the  isometric  system.  The  char- 
acteristic of  aggregates  lies  in  the  fact  that  the  arrangement  of  the 
more  or  less  regularly  bounded  individuals,  which  are  crowded  to- 
gether as  an  aggregation,  is  neither  parallel  nor  symmetrical.  This 
irregular  crystallographic  arrangement  causes  the  optical  orientation  to 
vary  with  each  individual  grain  of  the  aggregate.  In  such  an  aggrega- 
tion, when  viewed  between  crossed  nicols,  the  extinction  for  all  the 
individuals  can  never  be  in  the  same  azimuth  ;  they  will  show  different 
colors  or  different  degrees  of  light  and  shade,  which  will  depend  upon 
their  thickness,  the  position  of  the  thin  section  with  respect  to  their 
axes,  and  the  inclination  of  their  principal  plane  to  those  of  the  nicols. 

This  optical  appearance  of  aggregates  between  crossed  nicols  is 
called  aggregate-polarization  (PI.  VIII.  Figs.  4  and  5),  in  distinction 


SPHERICAL  AGGREGATES.  89 

from  the  optical  behavior  of  crystals,  which  is  uniform  throughout 
their  whole  extent.  The  boundaries  of  the  individuals  forming  an 
aggregate,  which  in  ordinary  light  often  are  scarcely  noticeable,  are 
very  marked  between  crossed  nicols,  and  show  the  manner  of  arrange- 
ment or  the  texture  of  the  aggregate. 

Spherical  aggregates,  so  common  in  the  mineral  kingdom,  deserve 
special  notice.  Of  these  the  spherulites  (Sphcerocrystals)  already  men- 
tioned are  a  particular  case.  They  consist  sometimes  of  a  singly  refract- 
ing amorphous  substance  ;  at  others  of  a  crystalline  mass  arranged  in 
concentric  shells,  or  in  radial  fibres  ;  sometimes  both  forms  of  arrange- 
ment occur  together,  so  that  the  spheres  consist  of  concentric  shells 
wrhich  in  turn  are  made  up  of  individuals  set  at  right  angles  to  the 
shells.  More  rarely  there  are  spherical  aggregates  in  which  both  radial 
and  concentric  arrangement  is  wanting. 

Radial  and  concentric  aggregates  occur  with  the  most  different 
minerals,  as  for  instance  calcite  and  other  carbonates,  Quartz,  chlorite, 
dellesite,  feldspar,  etc. 

If  one  considers  a  spherical  aggregate  of  an  amorphous  substance 
that  is  built  up  of  concentric  shells  each  of  which  exerts  a  pressure 
on  all  those  within  it,  then  the  density  of  the  sphere  will  increase 
toward  its  centre.  Such  a  sphere  may  be  considered  as  composed  of 
radial  cylinders  in  which  the  elasticity  in  the  direction  of  the  axis  of 
the  cylinder  is  greater  than  at  right  angles  to  it,  that  is,  as  a  radially 
fibrous  aggregate  of  optically  uniaxial,  negative  crystals. 

A  central  cross-section  through  such  a  sphere,  or  through  one  made 
up  of  orthorhombic  crystals,  when  viewed  in  parallel  polarized  light 
between  crossed  nicols  is  divided  into  four  light  quadrants  separated 
by  a  dark  cross,  whose  arms  are  at  right  angles  to  one  another  and 
parallel  to  the  principal  planes  of  the  nicols.  On  rotating  the  section 
through  a  complete  circle  the  actual  position  of  the  cross  does  not 
change  with  respect  to  the  planes  of  the  nicols,  though  it  appears  to 
rotate  in  opposite  direction  to  the  rotation  of  the  section.  The  lightest 
part  of  the  quadrants  is  along  the  radii  inclined  45°  to  the  principal 
planes  of  the  nicols,  from  which  it  diminishes  gradually  on  both  sides 
to  complete  extinction  along  the  radii  parallel  and  perpendicular  to 
these  principal  planes  (PI.  IX.  Figs.  1  and  2).  The  cross  shades 
gradually  into  the  light  quadrants.  If  the  sphere  consists  of  an  amor- 
phous substance,  and  its  double  refraction  is  the  result  of  centripetal 
condensation,  then  the  color  of  the  quadrants  will  diminish  from  the 
centre  outward,  which  is  not  the  case  with  a  proper  spherulite. 
Such  amorphous  spheres  may  therefore  show  colored  rings  in  parallel 


90  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

polarized  light  under  certain  conditions.  If  the  analyzer  be  rotated 
until  it  comes  into  parallel  position  with  the  polarizer,  the  dark  cross 
will  gradually  open  until  a  white  one  at  last  replaces  it,  when  the  light- 
colored  quadrants  will  appear  in  their  complementary  colors. 

Sections  which  have  not  been  cut  exactly  through  the  centre  of 
such  spherical  aggregates  show  the  same  phenomena  in  a  less  precise 
form. 

If  the  individuals  of  a  radial  aggregate  are  not  grouped  about  a 
point,  but  along  a  line  or  plane,  there  arise  distorted  splierulites, 
which  Zirkel  has  called  axiolites,  whose  dark  cross  between  crossed 
nicols  can  only  be  closed  in  four  definite  positions,  which  are  at  right 
angles  to  one  another,  while  on  rotating  the  section  the  cross  must  be 
open  or  be  resolved  into  two  hyperbolas  in  every  other  position. 

Radial  aggregates  of  monoclinic  or  asymmetric  crystals  might  pre- 
sent the  same  phenomenon  except  that  the  arms  of  the  cross  would  not 
in  general  lie  parallel  to  the  principal  plane  of  the  nicols,  but  would  be 
inclined  to  it  at  an  angle  depending  on  the  position  of  their  principal 
optical  plane  with  reference  to  the  direction  of  the  rays  of  crystals. 
But  the  cross  would  have  four  arms  at  right  angles  to  one  another  only 
for  the  case  in  which  all  the  needles  within  the  section  plane  had  the 
same  crystallographic  and  optical  orientation  with  respect  to  that 
plane ;  for  if  the  needles  were  variously  rotated  about  their  axes,  they 
would  give  rise  to  a  many-armed  cross  whose  arms  would  be  irregular 
in  size  and  position.  Homogeneous  spherical  aggregates  of  such 
crystals  are  only  known  at  present  for  certain  triclinic  feldspars 
(oligoclase  in  variolites). 

Spherical  aggregates  of  amorphous  substances  not  subjected  to 
strain  appear  dark  in  all  positions  between  crossed  nicols,  while 
spherical  aggregates  of  crystallized  substances  in  which  all  the  individ- 
uals lie  parallel  to  one  another  must  behave  like  simple  crystals,  being 
dark  in  four  positions  of  rotation  at  right  angles  to  one  another,  arid 
light-colored  in  all  other  positions.  Such  spherical  aggregates  may  be 
intergrown  with  more  or  less  amorphous  material  without  the  phenom- 
ena changing. 

Spherical  aggregates  composed  of  granular  individuals,  whose 
dimensions  may  be  greater  at  the  centre,  or  the  periphery,  are  called 
granospherites  (PI.  IX.  Fig.  3)  in  contradistinction  to  radial  and 
shelly  spherulites. 


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III.  CHEMICAL  PROPERTIES. 
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—  Contributions  a  1'etude  des  proprietes  physiques  et  ckimiques  des  mineraux  micro- 

scopiques.     Inaug.-Diss.  Paris.  1880. 
M.  WEBSKY,  Die  Mineralspecies  nach  den  fur  das  specifische  Gewiclit  derselben 

angenommenen  und  gefundeuen  Werthen.     Breslau.  1868. 
L.  VAN  WERVEKE,  Ueber  Regeneration  der  Quecksilberjodidlosung  und  liber  einen 

einfachen  Apparat  zur  mech.  Trennung  mittelst  dieser  Losung.   K  J.  B.   1883. 

II.  86. 

A  chemical  investigation  of  the  mineral  constituents  of  a  rock  may 
be  undertaken  not  only  for  the  purpose  of  confirming  an  optical  diag- 
nosis, but  may  be  necessary  in  many  cases  in  order  to  determine  the 
particular  species  within  a  family,  or  to  take  the  place  of  an  optical 
determination,  when  this  is  insufficient,  as  for  opaque  or  isometric 
minerals.  From  the  nature  of  microscopical  investigations  it  often 
happens  that  the  chemical  methods  used  in  mineral  analysis  are  not 
serviceable.  The  small  quantities  to  which  it  is  necessary  to  apply  the 
reagents  require  an  unusual  sharpness  in  the  reactions  ;  the  impossibil- 
ity of  distinguishing  colorless  and  amorphous  precipitates  micro- 
scopically determines  the  use  of  only  those  reactions  which  furnish 
characteristically  distinct  coloration  or  easily  recognized  crystalliza- 
tions. In  general  those  methods  are  to  be  preferred  which  furnish 
crystallizations  that  are  independent  of  the  relative  proportion  of  the 
substances  taking  part  in  the  reaction,  and  also  of  the  physical  con- 
ditions under  which  the  experiment  takes  place. 

The  reactions  given  in  the  following  pages  may  all  be  carried  on 
with  easily-devised  apparatus  and  under  the  ordinary  microscope.  The 
chemical  tests  may  be  made  on  the  thin  sections  themselves,  or  on  the 
minerals  which  have  been  isolated  from  the  rock  mechanically.  In  the 
first  case  uncertainties  might  often  arise  as  to  which  constituent  took 
part  in  the  reaction ;  but  this  uncertainty  may  often  be  entirely  obvi- 
ated. On  the  other  hand,  there  are  particular  reactions  which  can 
scarcely  be  carried  on  except  on  a  thin  section,  and  so  the  testing  of 
the  isolated  powder  must  be  supplemented  by  that  of  the  section. 


DETECTION  OF  CARBONIC  ACID.  93 

I 

Chemical  Investigation  of  Thin  Sections. 

Thin  sections  which  are  intended  primarily  for  chemical  investiga- 
tion should  be  left  uncovered,  and  it  is  better  not  to  polish  their  upper 
surface,  as  the  surface  exposed  to  the  reagents  will  then  be  greater  and 
the  chemical  action  more  energetic.  If  only  a  part  of  the  section  is 
to  be  tested,  this  is  separated  from  the  rest  by  a  thread  of  viscous 
balsam  in  the  form  of  a  ring,  which  prevents  the  drop  of  the  reagent 
from  spreading ;  the  latter  may  be  applied  through  a  capillary  pipette. 
If  an  already  covered  section  is  to  be  investigated,  the  glass  cover  is 
removed  by  a  knife-edge,  and  the  balsam  washed  off  with  a  brush 
dipped  in  alcohol  or  ether.  If  only  a  part  of  the  section  is  to  be  tested, 
the  glass  covering  this  part  may  be  carefully  cut  across  with  a  dia- 
mond, the  glass  cover  removed  as  before,  and  the  section  cleaned  in 
the  same  way.  If  the  portion  of  the  section  to  be  investigated  is  very 
small,  and  the  reagent  should  not  touch  any  other  part,  then  the  glass 
cover  may  be  entirely  removed  and  replaced  by  one  in  which  a  fine 
funnel-shaped  hole  has  been  bored.  When  the  opening  is  properly 
adjusted  over  the  right  spot,  the  balsam  is  heated  and  the  cover  made 
fast.  The  balsam  in  the  opening  is  removed  with  alcohol.  The 
reagent  is  then  confined  to  the  spot  beneath  the  opening.  Glass 
covers  may  be  prepared  beforehand  for  such  purposes  by  covering 
them  with  wax,  and  after  a  small  circle,  0.5-1  mm.  in  diameteiyhas 
been  cleaned  in  the  proper  place,  subjecting  them  to  hydrofluoric  acid 
until  they  are  eaten  through.  If  the  acids  to  be  used  on  the  section 
would  attack  glass,  perforated  platinum  foil  may  be  used  instead. 
The  treatment  of  thin  sections  with  weaker  or  stronger  acid  serves  to 
detect  or  remove  easily  soluble  constituents,  to  distinguish  gelatinizing 
silica  from  non-gelatinizing,  or,  finally,  to  produce  etched  figures  on 
minerals. 

To  the  more  or  less  easily  soluble  minerals  which  are  widely  distrib- 
uted in  rocks  belong  the  carbonates,  phosphates,  and  many  iron  ores. 
Upon  the  solution  of  the  carbonates,  of  which  calcite  is  soluble  in  acetic 
acid,  others  in  cold  hydrochloric  acid,  and  still  others  only  in  hot  acid, 
there  occurs  an  effervescence  through  the  escape  of  carbonic- acid  gas 
which  will  not  elude  observation  except  for  very  small  amounts  of  the 
carbonate.  But  if  the  particles  of  carbonates  are  very  small  and  iso- 
lated in  the  section,  the  development  of  carbonic  acid  may  be  easily 
overlooked.  In  such  cases  it  will  be  well  to  cover  the  section  with 
water  and  a  glass  cover,  and  to  place  the  drop  of  acid  so  that  it  may 
diffuse  slowly  in  the  water  over  the  section.  With  a  low  power  the 


94:  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

formation  of  bubbles  of  carbonic  acid  may  be  observed  wherever  the 
carbonates  exist,  since  the  glass  cover  prevents  the  bursting  of  the 
bubbles.  If  the  section  must  be  warmed  during  the  operation,  it  may 
be  laid  on  a  perforated  copper  plate,  the  aperture  of  which  is  over  the 
diaphragm  of  the  stage  of  the  microscope,  and  the  necessary  tempera- 
ture may  be  obtained  by  heating  two  long  tongue-shaped  projections 
by  means  of  an  alcohol  lamp  or  a  gas  jet.  The  bases  with  which  the 
carbonic  acid  was  combined  are  found  in  the  solution  covering  the 
section.  This  may  be  taken  up  in  a  capillary  tube  and  transferred  to  a 
clean  object-glass,  and  the  bases  determined  by  the  ordinary  methods 
of  analysis,  or  by  those  to  be  given  later  on.  The  capillary  tubes 
should  be  kept  in  large  numbers  and  thrown  away  after  being  used 
once,  because  of  the  difficulty  of  cleaning  them.  In  many  cases  it  is 
well  to  carry  on  the  reaction  within  the  capillary  tube  itself  by  admit- 
ting the  solution  to  be  investigated  at  one  end  and  the  reagent  at  the 
other,  and  letting  them  act  on  one  another  within  the  tube. 

j  O 

Gas  may  be  generated  upon  the  solution  of  many  sulphides;  the 
gas  in  this  case  being  hydrogen  sulphide.  If  this  gas  has  been  gener- 
ated under  a  glass  cover,  it  may  be  detected  by  the  coloration  of  a 
strip  of  filter-paper  moistened  with  lead-water,  which  is  dipped  into 
the  solution  covering  the  section. 

Certain  phosphates  and  the  oxides  of  iron  and  manganese  dissolve 
in  mineral  acids  without  the  evolution  of  gas  ;  the  acid  mostly  em- 
ployed is  hydrochloric.  The  principal  phosphate  met  with  in  rocks  is 
apatite,  which  is  widely  distributed.  If  apatite  is  present  in  the 
section,  the  phosphoric  acid  may  be  detected  by  an  addition  of  ammo- 
nium molybdate.  If  the  test  is  applied  directly  to  the  apatite,  it  is 
better  not  to  treat  the  section  with  hydrochloric  acid,  but  to  use  a 
drop  of  ammonium  molybdate  which  is  dissolved  in  nitric  acid.  After 
the  action  has  been  completed  the  solution  is  put  upon  a  clean  object- 
glass,  and  there  forms,  sometimes  after  a  slight  warming,  a  great  amount 
of  very  small  crystals,  mostly  resembling  rhombic  dodecahedrons,  which 
are  greenish  in  transmitted  light  and  yellow  by  incident  light ;  they 
occur  sometimes  singly,  sometimes  united  in  more  or  less  regular 
groups  (PI.  'XIII.  Fig.  5).  If  the  rock  contains  silicates  which  are 
easily  attacked  by  acids,  the  phosphoric  acid  cannot  be  determined  in 
the  manner  just  given,  since  soluble  silica  gives  a  similar  reaction  with 
ammonium  molybdate.  In  this  case  the  solution  obtained  by  diluted 
nitric  acid  must  be  evaporated  on  the  object-glass ;  and  after  sufficient 
heating,  by  which  the  silica  passes  into  the  insoluble  state,  it  is  again 
brought  into  solution  and  the  reagent  applied. 


GELATIX1ZAT10N.  (V\r  '  95 

\v  ^*       ^i 

Among  the  iron  oxides  limonite  is  the  most  readily  soluble  in 

hydrochloric  acid,  then  magnetite;  and  hematite  and  ilmenite  with 
the  most  difficulty.  They  all  dissolve  more  slowly  in  thin  section 
than  in  powder  because  of  the  smaller  surface  attacked.  Chromic  iron 
is  insoluble  or  nearly  so :  therefore  a  thin  section  is  rarely  treated 
with  hydrochloric  acid  for  the  purpose  of  distinguishing  these  iron 
ores ;  more  frequently  it  is  necessary  to  remove  them  by  acids  in  order 
to  observe  minerals  or  structural  relations  which  they  conceal.  This  is 
often  necessary  with  porphyritic  rocks  and  clay  slates  or  phyllites.  To 
test  for  the  presence  of  native  iron  in  a  thin  section,  it  is  covered  with 
a.  solution  of  copper  sulphate  from  which  there  is  deposited  on  the 
metallic  iron,  if  present,  a  coating  of  metallic  copper.  In  order  to 
avoid  confusion  with  a  coating  of  rust,  A.  von  Lasaulx  recommended 
the  use  of  the  solution  of  cadmium  borotungstate  employed  for  the  me- 
chanical separation  of  minerals,  this  becomes  deep  violet-blue  through 
reduction  in  the  vicinity  of  metallic  iron.  Zinc  and  copper  have  the 
same  action,  and  therefore  should  not  be  present. 

The  treatment  of  thin  sections  with  acids  is  to  be  specially  recom- 
mended for  proving  the  presence  of  gelatinizing  silica.  The  method 
of  procedure  is  governed  by  the  object  in  view.  If  it  is  only  a  ques- 
tion of  the  presence  of  such  silicates  as  belong  to  the  family  of  olivine, 
nepheliue,  zeolite,  the  more  basic  feldspars,  chlorite,  or  serpentine, 
then  the  carefully  cleaned  section  is  covered  with  a  thin  coating  of  the 
acid  employed.  If  the  layer  of  fluid  on  the  section  is  too  thick,  the 
resulting  gelatine  spreads  itself  over  the  whole  section,  and  gives  those 
portions  which  have  not  gelatinized  the  appearance  of  having  been 
attacked.  When  the  acid  has  acted  sufficiently  after  being  warmed,  it 
is  removed  by  rinsing  with  water>  and,  if  necessary,  with  the  addition 
of  a  drop  of  ammonia  to  neutralize  the  last  trace  of  acid.  The  action 
should  last  only  long  enough  to  form  a  very  thin  film  of  gelatinous 
silica  over  the  substances  attacked,  through  which  the  polarizing  phe- 
nomena of  the  minerals  may  be  observed.  So  it  is  better  to  repeat  a 
test  several  times  than  to  permit  it  to  work  too  strongly  the  first  time. 
In  order  to  render  the  transparent  gelatinous  silica  more  apparent,  the 
section  is  covered  with  a  drop  of  water  to  which  a  dilute  solution  of 
fuchsine  in  water  has  been  added,  and  is  allowed  to  stand  for  some 
time.  In  this  way  the  gelatinous  film  is  saturated  with  the  pigment ; 
the  section  is  then  rinsed  thoroughly  with  water,  when  the  color  disap- 
pears from  all  places  except  those  in  which  a  gelatinization  has  taken 
place.  If  the  acid  has  not  attacked  the  mineral  sufficiently,  the  fuch- 
sine is  destroyed  bv  a  drop  of  acid  and  the  experiment  repeated.  Any 

i 


96  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

other  coloring  material,  which  will  be  absorbed  by  the  gelatinous 
silica,  may  be  used.  In  many  cases  it  is  well  to  cover  the  partially 
gelatinized  section  with  the  solution  of  a  salt,  as  an  iron  salt,  and  after 
this  has  penetrated  sufficiently  into  the  gelatine,  to  add  a  reagent 
which  will  produce  a  coloring  or  precipitate  in  the  imbibed  solution 
of  salt  (ferrocyanide  of  potassium  or  ammonia).  This  method  is  espe- 
cially recommended  for  permanent  preparations. 

If  one  wishes  to  determine  the  bases  which  were  present  in  the 
gelatinized  substances,  the  acids  are  allowed  to  act  longer  and  more 
strongly.  The  solution  is  then  removed  with  a  capillary  pipette,  is 
evaporated  on  an  object-glass  in  order  to  render  the  dissolved  silica  in- 
soluble, is  again  dissolved  in  acidulated  water,  and  tested  by  methods 
to  be  described  later  on. 

Finally,  if  it  is  desired  simply  to  remove  the  gelatinized  substances 
(in  the  case  of  zeolitic  decomposition  of  feldspar  rocks,  of  chloritic  and 
serpentinous  alteration  of  pyroxenes  and  amphiboles,  etc.),  then  they 
are  destroyed  as  completely  as  possible,  and  the  section  is  thoroughly 
rinsed  with  a  strong  jet  of  water,  in  order  to  wash  off  the  gelatinous 
silica,  which  often  adheres  stubbornly.  Minerals  are  sometimes  dis- 
covered in  this  way  whose  presence  would  scarcely  be  suspected  on 
observing  the  sections  before  they  were  attacked. 

Etched  figures  have  been  of  much  less  service  in  the  microscopical 
investigation  of  mineral  aggregates  and  rocks  than  in  the  physical 
researches  in  crystallography,  because  of  the  uncertain  position  of  the 
sections  of  minerals  composing  aggregates,  and  the  essential  depend- 
ence of  the  symmetry  of  the  etched  figures  on  that  of  the  crystal  face 
on  which  they  have  been  produced.  Nevertheless  they  may  often  be 
employed  to  advantage  for  determining  tfie  presence  of  twinning,  to 
prove  the  law  of  twinning  derived  from  the  apparent  form,  or  to 
furnish  evidence  of  the  parallel  growth  of  closely  related  minerals 
which  belong  to  different  systems.  Etched  figures  furnish  definite 
conclusions,  especially  in  the  study  of  minerals  of  the  pyroxene  and 
amphibole  families,  when  other  methods  leave  one  in  doubt.  They 
may  also  be  advantageously  employed  in  the  investigation  of  the  ori- 
entation of  the  optical  ellipsoid  of  elasticity  in  minerals  of  the  mica, 
chloritoid,  and  chlorite  series,  when  their  outward  form  is  wanting. 
Finally,  etched  figures,  in  certain  cases,  give  criteria  for  the  determi- 
nation of  substances  which  otherwise  are  distinguishable  with  diffi- 
culty, as,  for  example,  quartz  and  cordierite,  when  there  are  no  sections 
which  permit  a  positive  optical  determination. 

Etched  figures  are  obtained  by  the  use  of  various  acids,  or  of 


ETCHED  FIG  URES.  97 

caustic  alkali,  according  to  the  substance  under  investigation,  which 
also  determines  the  conditions  under  which  the  acids  are  allowed  to 
act.  The  action  should  be  as  gentle  as  possible  to  produce  sharp  • 
figures  whose  form  and  symmetry  may  be  plainly  recognized.  After 
the  corrosion  of  the  reagent  the  etched  substance  is  thoroughly  freed 
from  compounds,  which  may  .have  resulted  from  the  reaction,  by  wash- 
ing in  water  or  acid,  and  the  thin  section  should  be  examined  in  a 
weakly  refracting  medium  (water).  If  it  is  placed  in  a  strongly  refract- 
ing medium,  the  etched  figures  may  be  completely  overlooked  unless 
strongly  divergent  light  is  transmitted  through  the  section  by  sinking 
the  condensing  lens.  In  every  case  the  objective  is  focused  on  the 
surface  of  the  section. 

The  forms  of  the  etched  figures  differ  on  one  and  the  same  face  of 
any  mineral,  according  to  the  corrosive  ngent  employed  ;  the  degree  of 
their  symmetry  alone  appears  independent  of  the  latter  and  of  its  con- 
centration. The  sharpest  etched  figures  are  produced  on  crystal  faces 
and  cleavage  planes ;  they  are  only  moderately  precise  and  clear  on 
artificially  made  faces  (ground  faces)  when  these  are  well  polished. 

Heating  thin  sections  to  a  red  heat  serves  to  reveal  hydrous  min- 
erals and  carbonaceous  substances,  or  to  produce  colorations  which  are 
characteristic  of  certain  compounds.  Most  hydrous  minerals,  such  as 
zeolites  and  chlorites,  become  clouded  through  the  high  heating  of 
thin  sections  containing  them.  For  this  purpose  the  section  is  re- 
moved from  the  object-glass,  carefully  cleaned  of  balsam  by  means  of 
alcohol  or  ether,  and  brought  into  the  flame  on  thin  platinum  foil. 
Colorless  hydrous  minerals  simply  become  clouded;  colored  ones 
change  their  color ;  chloritic  substances  upon  sufficient  heating  become 
rust-brown  or  black.  Carbonaceous  particles  scattered  through  a  sec- 
tion may  be  distinguished  from  those  of  iron  oxide  by  heating  to  red- 
ness, by  which  process  the  carbonaceous  matter  is  consumed.  As 
these  two  sometimes  occur  mechanically  combined,  it  is  well  in  free- 
ing a  section  of  such  impurities  to  alternate  the  processes  of  treating 
with  acid  and  of  heating  to  redness.  The  combustion  of  carbonaceous 
substances  varies  greatly  ;  in  many  cases  graphite  is  not  consumed  even 
by  continued  and  strong  heating. 

Colorless  silicates  containing  protoxide  of  iron  are  colored  red  and 
reddish  brown  on  being  heated  to  redness.  C.  W.  C.  Fuchs  first 
observed  this  property  in  olivine.  Pyroxene  and  hornblende,  when 
colorless  or  only  faintly  colored,  act  in  the  same  manner.  Olivine 
sometimes  becomes  pleochroic ;  hornblende  always  so,  and  often  extra- 
ordinarily strong.  With  the  latter  mineral  the  colors  and  pleochro- 


98  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

ism  are  the  same  as  those  in  the  hornblende  of  rock  inclusions  in 
lavas  and  volcanic  ejectamenta.  The  phenomenon  may  be  referred 
to  an  extremely  fine  distribution  of  sesquioxide  of  iron  freed  from  com- 
bination. H.  Vogelsang  showed  that  minerals  of  the  haiiyne  group 
may  become  blue  upon  being  heated,  if  they  did  not  already  possess 
this  color. 

Colorless  aluminous  minerals  are  colored  blue  if  the  section  is 
moistened  with  very  dilute  cobalt  solution  on  platinum-foil,  is  very 
strongly  heated,  and  then  digested  with  dilute  hydrochloric  acid.  To 
increase  the  temperature  sufficiently  it  is  covered  with  a  platinum 
cover.  Often  the  reaction  only  takes  place  after  repeated  heating. 

Microchemical  Investigation  of  Loose  Grains. — Preparation  of  the 
Material  for  Observation. 

In  order  to  investigate  the  constituents  of  a  mineral  aggregate  or 
of  a  rock  in  a  pure  condition  it  is  necessary  to  separate  them  from  the 
mixture.  The  separation  of  a  mixture  into  its  mineral  components 
is  seldom  effected  by  the  successive  application  of  a  single  method. 
Generally  several  methods  must  be  used  in  connection  with  one  an- 
other, which  are  based  partly  upon  the  different  specific  gravities  of 
the  constituents,  partly  on  their  different  susceptibility  to  chemical 
reagents,  and  partly  on  their  behavior  towards  stronger  or  weaker  mag- 
nets. For  all  these  kinds  of  separation  it  is  necessary  to  bring  the  mix- 
ture into  the  form  of  a  powder,  and  to  give  the  powder  not  only  such 
dimensions  that  each  grain  or  the  greater  number  of  them  shall  be 
homogeneous — that  is,  shall  consist  only  of  one  kind  of  mineral — but 
the  size  of  the  grains  must  be  as  uniform  as  possible.  The  coarseness 
of  a  powder  in  a  given  case  depends  on  the  grain  of  the  mixture,  for 
the  grain  of  the  powder  should  be  as  large  as  possible,  since  the  sepa- 
ration is  easier  and  more  successful  the  larger  the  grain  of  the  powder 
to  be  separated.  The  finer  the  powder  is,  the  slower  and  more  diffi- 
cult will  be  the  mechanical  separation,  but  the  quicker  and  easier  the 
chemical.  It  is  very  desirable  that  the  grains  should  not  lose  their 
crystal  form,  if  they  possess  any,  and  that  in  the  absence  of  crystal 
form  their  boundary  should  be  made  up  of  cleavage  faces.  This  ob- 
ject is  best  accomplished  by  reducing  the  material  in  a  metal  mortar, 
by  striking  it  with  the  pestle  and  avoiding  the  rubbing  and  grinding  of 
the  powder  as  far  as  possible.  When  the  proper-sized  grain  has  been 
approximately  reached,  the  powder  is  separated  into  portions  of  like- 
sized  grain  by  means  of  a  series  of  fine  wire  sieves  with  meshes  of 


TLLO  ULET'S  1SOL  VTION.  99 

about  1  to  0.2  sq.  mm.  In  place  of  the  wire  sieves,  a  series  of  sieves 
may  be  made  by  covering  one  end  of  a  number  of  wooden  or  tin 
cylinders  with  different  grades  of  bolting-cloth,  held  in  place  by  tightly 
fitting  rings,  which  project  far  enough  below  the  bottom  of  one  sieve 
to  fit  over  the  top  of  the  next,  and  so  form  a  closed  set  of  boxes 
which  can  be  shaken  together.  The  different  grades  of  powder  within 
these  boxes  are  examined  microscopically  to  see  which  furnishes  the 
requisite  homogeneity  of  the  single  grains  ;  the  whole  powder  is  then 
reduced  to  this  size  of  grain,  and  is  put  in  a  large  vessel  and  washed 
free  of  the  fine  mineral  dust  which  remains  suspended  in  the  water. 
For  chemical  separation  this  is  not  necessary. 

The  order  in  which  the  separations  by  specific  gravity,  by  magnets, 
•or  by  chemical  action  are  to  be  applied  to  a  powder  will  depend  on 
the  problem  presented  in  each  particular  case. 

Separation  according  to  Specific  Gravity. — An  actual  separation 
of  a  mixed  powder  according  to  the  specific  gravity  of  its  constitu- 
ents is  only  obtained  by  the  use  of  fluids  which  are  heavier  than  the 
powder,  so  that  it  floats  on  the  fluid,  and  which  can  be  diluted  by  the 
addition  of  lighter  fluids  and  made  specifically  lighter.  The  fluids 
most  generally  employed  are  the  so-called  Thoulet's  and  Klein's 
solutions. 

Thoulet's  solution  was  first  proposed  by  E.  Sonstadt  and  afterwards 
by  Church,  but  became  generally  known  through  the  researches  of  Thou- 
let,  and  was  thoroughly  investigated  by  Y.  Goldschmidt.  It  is  a  solu- 
tion of  potassium-mercuric  iodide,  whose  maximum  density,  according 
to  Y.  Goldschmidt,  is  3.196.  According  to  Goldschmidt's  statement, 
the  highest  specific  gravity  is  obtained  when  a  mixture  of  mercuric 
iodide  is  dissolved  in  cold  water  with  potassium  iodide  in  the  propor- 
tion KI  :  Hgl,  =  1  :  1.24,  and  this  solution  is  evaporated  on  the  water- 
bath  until  a  crystalline  coat  forms  on  the  surface,  or  until  a  crystal  of 
tourmaline  or  fluorite  floats  on  it  (sp.  gr.  —  3.1).  Upon  cooling,  the 
density  of  the  solution  rises  to  3.196  through  contraction.  According 
to  van  Werveke's  observations,  an  excess  of  KI  does  no  harm.  Upon 
filtering,  the  solution  is  perfectly  transparent,  and  of  a  yellowish-green 
color.  So  long  as  the  relation  of  the  two  salts,  KI  and  HgI2,  is  cor- 
rect, the  solution  may  be  continually  diluted  as  far  as  a  sp.  gr.  1.0,  and 
by  evaporation  on  a  water-bath  be  brought  back  to  a  maximum  3.196. 
If  the  relation  of  the  salts  is  changed,  then  with  an  excess  of  Hgla 
there  separates  out  a  yellow  hydrous  double  salt  in  acicular  crystals ; 
with  an  excess  of  KI  this  substance  separates  in  cubes.  The  same 
separations  take  place  when  the  solution  stands  a  long  time  in  dry  air. 


100  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

Through  long  usage,  the  solution  loses  its  green  color  and  becomes  red- 
dish brown  from  the  separation  of  iodine.  One  may  avoid  this 
decomposition,  or  bring  the  altered  solution  back  to  its  original  condi- 
tion by  adding  metallic  mercury  during  evaporation.  The  free  iodine 
then  combines  with  the  mercury  to  form  mercurous  iodide,  which 
coats  the  metallic  mercury  with  a  fine  grayish-green  dust,  and  causes 
it  to  fall  apart  in  small  spherules  upon  being  stirred  up ;  these  unite 
only  with  great  difficulty.  Upon  further  evaporation  Hgl  is  changed 
to  HgI2  and  Hg,  and  the  mercuric  iodide  combines  with  the  excess  of 
potassium  iodide.  The  solution  changes  in  the  air  through  the  giving 
off  and  taking  up  of  water,  and  in  this  way  its  density  is  altered.  Separa- 
tions in  this  solution,  therefore,  must  be  carried  on  with  constant  tem- 
perature or  in  closed  vessels,  and  with  the  greatest  possible  dispatch. 
Its  specific  gravity  is  nearly  constant  when  it  is  about  3.01-3.1 ;  be- 
low this  limit  it  increases  by  losing  water,  and  above  it  it  decreases  by 
taking  up  water.  Consequently  the  concentrated  solution  may  be  ex- 
posed to  the  air  without  its  altering  noticeably. 

The  dilution  of  the  concentrated  solution  to  a  particular  specific 
gravity  by  the  addition  of  water  cannot  be  accomplished  with  cer- 
tainty by  the  introduction  of  a  measured  amount  of  water,  because  of 
the  contraction  which  takes  place.  One  must  proceed,  therefore,  em- 
pirically, and  place  in  the  solution  a  piece  of  mineral  of  the  required 
specific  gravity  as  an  indicator,  and  then  proceed  to  add  water  very 
carefully,  drop  by  drop,  or,  for  a  small  difference  between  the  initial 
and  desired  density,  add  a  dilute  solution  until  the  indicator  is  sus- 
pended in  the  solution. 

Since  metallic  iron  decomposes  the  solution  with  the  separation  of 
mercury,  all  splinters  from  the  mortar  which  might  have  gotten  into 
the  powder  must  be  removed  by  a  magnet  or  by  acid,  before  putting 
the  powder  in  the  solution. 

The  solution  discovered  by  Dr.  Klein,  and  named  after  him,  is  that 
of  a  cadmium  borotungstate,  with  the  formula  2H2O,  2CdO,  B3O3, 
9WoO3  +  16aq.  This  salt  dissolves  at  22°  C.  in  less  than  10  times  its 
weight  of  water ;  the  light  yellow-colored  solution  has  a  specific  gravity 
of  3.28  at  15°  C.  If  a  diluted  solution  of  this  salt  is  evaporated  on  the 
water-bath,  the  violet  color  which  is  frequently  observed  disappears  as 
soon  as  the  sp.  gr.  2.7  is  reached.  If  the  evaporation  is  continued 
until  an  augite  crystal  floats  on  the  warm  solution,  crystals  are  formed 
upon  its  cooling,  which,  when  dissolved  in  a  little  water,  yield  a  solu- 
tion in  which  olivine  will  float ;  by  combining  these  two  solutions  one 
is  obtained  with  sp.  gr.,  3.3-3.6.  The  highest  possible  specific  gravity, 


KLEIN'S  SOLUTION.  101 

3.6,  is  obtained  by  evaporating  on  a  water-bath  until  olivine  floats  on 
the  warm  solution.  Cadmium  borotungstate  is  deposited  in  crystalline 
masses  which  consist  of  rhombic  individuals.  If  these  are  cleaned  by 
drawing  off  as  much  of  the  mother-liquor  as  possible,  and  are  then 
heated  in  a  tube  in  the  water-bath,  they  melt  at  75°  C.  in  their  water  of 
crystallization,  and  form  a  somewhat  mobile  fluid,  on  which  spinel 
floats.  This  concentration  may  also  be  reached  by  the  evaporation  of 
the  solution  on  the  water- bath.  At  a  very  high  specific  gravity  the 
Klein  solution  is  quite  oily,  and  its  applicability  for  the  separation  of 
powder  is  very  limited,  and  only  coarse  powder  can  be  separated  by  it. 
By  evaporating  the  dilute  solution  until  a  crystalline  coating  is  formed, 
and  after  its  subsequent  filtration,  a  cold  solution  is  obtained  with 
sp.  gr.,  3.36-3.365,  which  is  generally  serviceable.  This  solution, 
like  Thoulet's,  is  miscible  with  water  under  all  conditions  without 
decomposition. 

It  has  the  advantage  of  higher  specific  gravity  and  of  being  innox- 
ious, but  its  preparation  is  far  less  simple  than  that  of  Thoulet's  solu- 
tion. The  solution  is  decomposed  by  metallic  iron,  zinc,  and  lead,  as 
well  as  by  carbonates.  Consequently  these  substances  must  be  re- 
moved from  the  powders  with  acids  before  they  come  in  contact  with 
the  solution. 

C.  Rohrbach  suggested  the  use  of  a  solution  of  barium  mercuric 
iodide,  which  with  proper  treatment  reaches  a  sp.  gr.,  3.588,  and  is  still 
quite  mobile.  The  solution,  however,  cannot  be  diluted  with  water 
without  being  decomposed,  which  prevents  its  general  application. 
It  is  only  employed  in  cases  where  the  specific  gravity  is  above  that 
of  Thoulet's  and  Klein's  solutions,  and  where  a  separation  cannot  be 
made  by  chemical  or  magnetic  methods. 

K.  Bran  us  *  has  recently  suggested  the  use  of  methyl  iodide  for 
separating  minerals  with  high  specific  gravity.  Methyl  iodide,  CH2I2, 
is  a  yellow  fluid,  strongly  refracting  and  very  mobile.  It  is  easily  mis- 
cible with  benzole,  but  not  with  water  or  alcohol,  and  does  not  attack 
metallic  substances.  Its  specific  gravity,  which  at  16°  C.  is  3.3243, 
varies  considerably  with  the  temperature ;  thus  at  10°  C.  it  is  3.3375, 
and  at  20°  C.  3.3155,  the  variation  being  about  0.0022  for  each  degree. 

A  solution  that  has  been  diluted  with  benzole  may  be  concen- 
trated by  evaporation  on  the  water-bath,  or,  if  there  is  only  a  small 
amount  of  benzole  present,  by  evaporation  in  a  draught.  The  con- 
centrated solution  does  not  change  upon  exposure  to  the  air,  which, 
together  with  its  high  index  of  refraction,— nna  —  1.74092  at  16°  C., 

*  N.  J.  B.  1886.  B.  II.  72. 


102  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


,  45 


renders  it  specially  useful  for  determining  indices  of  refraction  by 

means  of  total  reflection. 

The  vessels  used  for  mechanical  separations  are 
the  same  for  all  of  the  solutions  just  mentioned. 
They  have  been  made  with  a  variety  of  forms ;  but  the 
handiest,  most  solid,  and  most  convenient  form  is  that 
devised  by  T.  Harada,  represented  in  Fig.  45.  A 
long,  pear-shaped  vessel  of  thick  glass  is  closed  at 
the  upper  end  by  a  ground-glass  stopper,  and  at  the 
lower,  narrower  end  by  a  glass  cock.  The  solution 
and  powder  are  introduced  from  above,  the  stopper 
inserted,  and  the  mixture  vigorously  shaken  up. 
The  powder  is  then  allowed  to  rise  or  sink,  and  as 
soon  as  a  clear  stratum  of  the  fluid  appears  between 
the  upper  and  lower  portions,  a  small  glass  is  placed 
under  it  so  that  the  lower  end  of  the  apparatus  rests 
firmly  on  the  bottom  of  the  glass ;  the  cock  h  is  then 
opened.  A  small  part  of  the  solution  falls  out,  only  a 
few  drops,  until  the  pressure  of  the  air  balances  that  of 
the  column  of  fluid,  and  the  separation  of  the  heavier 
powder  which  falls  into  the  glass  proceeds  automatically.  One  must 
avoid  letting  an  air-bubble  into  the  narrow  part  of  the  apparatus. 
When  all  the  descending  powder  has  passed  the  cock  A,  it  is  closed, 
and  a  layer  of  water  is  put  over  the  solution  in  the  glass  ;  the  appara- 
tus is  raised  until  the  lower  end  reaches  the  layer  of  water.  The  water 
then  rises  up  to  the  cock  A,  and  allows  all  the  powder  beneath  it  to  fall 
into  the  glass.  The  further  dilution  of  the  solution  for  a  second  separa- 
tion of  the  powder  is  accomplished  by  adding  a  few  drops  from  above, 
or  better,  by  reversing  the  apparatus,  and  allowing  the  solution  in  the 
lower  part  of  it,  which  has  been  diluted  by  the  water,  to  enter  through 
the  opened  cock  h.  The  mixture  is  again  thoroughly  shaken  and  the 
operation  repeated. 

Harada's  separating  apparatus,  together  with  all  narrow  and  tube- 
shaped  apparatus,  has  the  disadvantage  that  the  heavier  powder  in  falling 
carries  down  with  it  mechanically  a  certain  part  of  the  lighter,  floating 
powder,  and  in  the  same  way  the  lighter  powder  holds  up  mechani- 
cally a  part  of  the  heavier;  and  also  that  in  the  space  between  the 
solution  and  the  stopper  a  mixed  powder  remains  sticking  to  the  walls 
of  the  vessel  in  consequence  of  the  shaking.  C.  W.  Brogger  sought 
to  obviate  this  by  modifying  Harada's  apparatus.  He  placed  in  the 
middle  of  the  vessel  a  wider-bored  cock  (Fig.  46«),  the  aperture  of 


SEPARATING  APPARATUS. 


103 


which  is  the  same  as  that  of  the  vessel.     Fig.  46#  shows  the  apparatus 
after  the  first  settling  of  the  heavier  powder  S»  with  the  middle  cock 


A  open.  The  powder  Sl  over  the  lower  cock  B  contains  a  part  of  the 
lighter  powder  $/ ;  the  lighter  powder  S9  at  the  top  contains  a  part  of 
the  heavier  $/.  If  the  cock  A  is  shut,  the  apparatus  shaken  vigor- 
ously and  inverted,  then  after  some  time  there  will  be  a  separation  of 
the  powder  in  both  parts  of  the  apparatus,  which  is  represented  in 
Fig.  465.  Now  if  the  apparatus  is  carefully  turned  in  the  position  46<?, 
the  heavy  powder  8l  and  $/,  and  the  lighter  powder  /St  and  $/  will 
move  in  the  directions  indicated  by  the  arrows  without  getting  mixed. 
If  this  movement  is  continued  until  $/  is  directly 
under  the  cock  A,  and  /S1  directly  above  it,  then  by 
carefully  opening  the  cock  the  powder  /$/  glides 
along  the  upper  side  of  the  apparatus  into  the  upper 
part,  while  the  powder  $/  descends  along  the  lower 
side  into  the  lower  portion.  The  cock  A  is  then 
closed  and  the  separation  completed,  the  heavier 
powder  St  and  $/  being  drawn  out  through  the 
lower  cock  B. 

It  is  well  in  any  case  to  repeat  the  separation 
several  times,  working  with  each  of  the  portions  of 
the  powder  separately.  In  treating  large  quanti- 
ties, especially  for  the  first  separation,  it  is  advisa- 
ble to  use  an  ordinary  separating  funnel,  with  a  stopcock  placed  some 


104 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


little  distance  below  the  funnel  proper  (Fig.  47).  The  mixture  is 
stirred  with  a  glass  rod. 

The  substances  to  be  separated  are  first  determined  optically  in 
order  to  know  approximately  the  specific  gravity  required  for  the  solu- 
tion. The  densities  of  the  minerals  most  widely  distributed  in  rocks 
are  given  in  a  table  on  page  110. 

In  order  to  bring  the  separating  solution  to  a  particular  density, 
which  lies  between  the  specific  gravity  of  the  bodies  to  be  separated,  it 
is  necessary  to  use  the  so-called  indicators  (or  floats),  or  to  employ  a 
balance  for  the  determination  of  the  specific  gravity  of  the  fluids.  As 
indicators,  one  may  use  mineral  fragments  whose  specific  gravity  has 
been  determined  with  the  utmost  exactness,  and  which  may  be  held  in 
readiness  in  great  numbers  in  glasses.  Y.  Goldschmidt  has  prepared 
such  a  scale,  with  convenient  intervals,  from  which  the  following  are 
selected  as  sufficient  for  ordinary  purposes: 


No.        Name.  Locality.           Sp.  gr. 

1.  sulphur Girgenti 2.070 

2.  hyalite Waltsch 2.160 

3.  opal .Scheiba 2.212 

4.  natrolite Brevig 2.246 

5.  pitchstone. . .  .Meissen 2.284 

6.  obsidian Lipari 2.362 

7.  pearlite Hungary 2.397 

8.  leucite Vesuvius  . . .    .2.465 

9.  adular St.  Gotthard.  .2.570 

10.  elseolite Brevig 2.617 


No.        Name.  Locality.  Sp.  £r. 

11.  quartz Middleville 2.650 

12.  labradorite.  .Labrador 2.689 

13.  calcite Rabenstein 2.715 

14.  dolomite Muhrwinkel 2.733 

15.  dolomite . . .  Kauris 2.868 

16.  prehnite  . . .  .Kilpatrick 2.916 

17.  aragonite.. .  .Bilin. 2.933 

18.  actinolite ....  Zillerthal 3.020 

19.  andalusite. .  .Bodenmais 3.125 

20.  apatite Ehrenfriedersdorf .  .3.180 


A  more  convenient  form  of  indicator  has  been  devised  by  "W".  H. 
Hobbs  :  it  consists  of  a  small  glass  tube,  closed  at  both  ends,  and  partly 
filled  with  some  heavy  metal,  which  should  be  confined  to  the  lower  end, 
so  that  the  glass  float  will  maintain  a  vertical  position  in  the  heavy  solu- 
tion. In  the  upper  end  is  inserted  a  loop  of  platinum  wire,  by  which 
it  may  be  readily  lifted  out  of  the  solution.  By  varying  the  weight  of 
metal  within  the  glass  tubes  a  series  of  indicators  may  be  made,  having 
any  desired  interval  between  them. 

If  the  solution  is  to  have  exactly  the  specific  gravity  of  one  of  the 
indicators  in  the  scale,  this  is  pk^ed  in  the  solution,  and  water  or  more 
concentrated  solution  is  added  until  the  indicator  is  suspended.  When 
this  remains  in  every  place  in  the  solution  in  which  it  is  brought,  the 
solution  is  adjusted,  and  has  exactly  the  desired  density.  Should  this 
lie  between  those  of  two  indicators  of  the  scale,  both  of  these  are 
placed  in  the  solution,  and  its  density  brought  to  a  state  in  which  the 
heavier  indicator  sinks  and  the  lighter  rises.  After  the  solution  has 
been  adjusted  the  mineral  indicators  are  lifted  out  with  a  glass  rod 
flattened  and  bent  at  one  end. 


WESTPHAL'S  BALANCE. 


105 


The  balance  for  determining  the  specific  gravity  of  fluids  (which  is 
made  by  G.  Westphal,  of  Celle,  province  of 
Hannover,  and  is  called  by  his  name)  is  better 
adapted  for  measuring  the  density  of  a  particu- 
lar solution  than  for  bringing  a  fluid  to  a  certain 
density.  This  measurement  may  be  easily  made 
even  during  a  separation,  if  a  separating  funnel 
be  used,  and  a  small  glass  tube  like  Fig.  48  be 
let  down  into  it.  This  tube  is  held  up  by  the 
three  arms  which  rest  on  the  edge  of  the  funnel,  and  is  filled  with 
solution  through  the  bent  tube  a  without  the  admission  of  the  powder. 
The  glass  sinker  of  the  balance  is  then  sunk  in  the  solution  within  the 
tube  r. 

Westphal's  balance  (Fig.  49)  consists  of  a  beam  /*,  suspended  on  a 
fulcrum  at  I,  which  terminates  behind  in  a  pointer  s,  whose  position 
can  be  read  from  a  scale  a.  For  a  horizontal  position  of  the  .bar  the 


•TW1 


pointer  s  must  be  at  the  zero-point  of  the  scale.  The  bar  /,  from  its 
point  of  suspension  to  the  small  hook  A,  is  divided  into  10  parts.  The 
support  t  of  the  beam  is  firmly  connected  with  the  scale  #,  and  together 
with  the  rod  p  sinks  in  the  hollow  cylinder  <?,  being  held  at  any  height 


106  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

by  the  screw  m.  The  hollow  cylinder  terminates  in  a  massive  foot 
resting  on  a  circular  base,  which  can  be  brought  into  a  horizontal  posi- 
tion by  means  of  the  screw  o.  From  the  hook  h  is  hung  the  sinker  ry 
which  is  immersed  in  the  fluid.  The  weights,  in  the  shape  of  riders, 
which  counteract  the  buoyancy  of  the  fluid,  and  restore  the  beam  to 
its  position  of  equilibrium,  are  hung  on  the  hook  A,  to  indicate  whole 
numbers.  The  riders  indicating  the  decimals  are  placed  on  the  beam 
/,  and  their  weight  read  from  the  scale  on  the  beam.  For  example,  if 
the  bar  swings  about  the  zero-point  of  the  scale  s  when  two  of  the 
riders  hang  from  the  hook  A,  the  rider  for  determining  the  first  decimal 
stands  at  8,  and  the  smallest  rider  for  determining  the  second  decimal 
is  half  way  between  4  and  5 ;  then  the  specific  gravity  of  the  fluid 
is  2.845. 

The  complete  success  of  a  mechanical  separation  according  to  specific 
gravity,  and  especially  the  exact  quantitative  separation  of  a  mixture, 
cannot  be  attained  because  of  the  following  hindrances:  (1)  The  im- 
possibility of  preparing  a  powder  which  shall  consist  of  nothing  but 
homogeneous  grains  ;  (2)  the  variations  in  the  specific  gravity  of  the 
constituents ;  (3)  the  change  of  specific  gravity  which  minerals 
experience  through  weathering,  decomposition,  and  alteration.  For 
these  reasons  one  must  be  satisfied  with  as  close  an  approximation  as 
possible  to  the  homogeneity  of  the  separated  powder.  Moreover,  every 
separation  is  accompanied  by  a  variable  amount  of  intermediate  prod- 
ucts, that  is,  mixtures  oi  several  minerals  grown  together,  together 
with  more  or  less  altered  grains  which  are  useless. 

Laminated  minerals,  such  as  mica,  often  float  much  longer  in  a 
solution  than  they  should,  according  to  their  specific  gravity,  and  there- 
fore render  all  subsequently  separated  portions  impure.  They  may  be 
removed  by  allowing  the  powder  to  glide  down  several  times  over 
paper,  by  which  means  the  laminated  minerals  remain  sticking  to  the 
paper ;  or  by  letting  the  powder  fall  in  small  portions  on  the  slightly 
moistened  sides  of  .a  funnel.  The  mica  plates  accumulate  on  the  sides, 
while  the  grains  of  other  minerals  roll  down  into  a  vessel  placed 

beneath. 

• 

Mechanical  Separation  of  a  Rock  Powder  by  means  of  the  Electro- 
magnet. 

As  magnetite  can  be  easily  extracted  from  a  mixed  mineral  powder 
with  an  ordinary  bar  magnet,  so  all  iron-bearing  minerals  may  be 
separated  from  non-ferruginous  minerals  by  means  of  an  electro- 
magnet. It  is  not  known  with  certainty  what  are  the  factors  which 


ELECTRO- MA  GNET.  107 

influence  the  attraction  of  a  mineral  by  an  electro-magnet,  since  it  is 
not  always  proportional  to  the  percentage  of  iron,  for  many  minerals 
rich  in  iron  (chromite,  biotite)  are  less  strongly  attracted  than  others 
much  poorer  in  iron.  Thus  the  attraction  of  a  mineral  can  often  be 
increased  by  heating  it  to  redness,  by  which  means  the  percentage  of 
iron  is  not  altered,  but  only  the  form  of  its  occurrence  is  changed. 
Minerals  of  the  amphibole,  pyroxene,  olivine,  epidote,  garnet,  and 
similar  series,  may  often  be  separated  with  an  electro-magnet  by  regu- 
lating the  magnetic  moment  of  the  electro-magnet.  It  may  also  be 
used  to  advantage  in  separating  individuals  rich  in  interpositions  from 
those  which  do  not  contain  them,  when  the  mineral  itself  is  free  from 
iron.  Thus,  it  is  easy  to  separate  the  leu  cite,  rich  in  inclusions  of  a 
rock  (Capo  di  Bove),  from  those  free  from  them  ;  the  brownish-clouded 
plagioclases  of  gabbros  also  may  be  completely  separated  from  the 
colorless  ones. 

The  magnetic  power  of  the  electro-magnet  is  best  regulated  by 
using  an  electro-magnet  in  the  shape  of  a  horseshoe,  on  whose  poles  are 
screwed  movable  plates  of  soft  iron  formed  as  in  Fig.  50.  Their 

wedge-shaped    projections    are   turned   towards     . 

one   another ;   the  magnetic  force  increases  very 
rapidly  the   closer  the  wedges   are   brought  to- 


gether,  and    diminishes    the    farther    they   are  Fig.  BO 

moved  apart.  The  plates  are  screwed  fast  at 
the  proper  distance  apart,  the  current  allowed  to  circulate  through 
the  magnet,  arid  the  powder  brought  near  the  edge  of  the  wedges 
on  a  piece  of  paper,  or  when  necessary  brought  in  contact  with 
them.  The  paper  with  the  powder  is  then  withdrawn,  and  the 
powder  which  was  attracted  is  allowed  to  fall  on  paper  spread  beneath 
it.  The  rapidly-repeated  opening  and  closing  of  the  circuit  is  most 
conveniently  accomplished  by  introducing  into  one  connecting  wire  a 
small  cup  of  mercury,  in  which  one  end  of  the  wire  can  be  dipped  and 
drawn  out  by  the  right  hand,  while  the  left  hand  is  manipulating  the 
powder. 

Not  infrequently  it  is  necessary  to  separate  single  grains  from  a 
mixture  by  hand,  when  the  other  methods  fail.  It  is  'then  well  to  use 
a  thick  strip  of  glass  in  which  a  longitudinal  groove  has  been  cut.  The 
powder  is  placed  in  this  groove  so  that  the  grains  may  not  lie  too  close 
together.  The  glass  plate  is  slowly  moved  along  under  the  microscope, 
and  as  soon  as  a  grain  of  the  mineral  sought  for  is  in  the  field,  it  is  re- 
moved with  a  thin  waxed  thread,  or  with  the  sharpened  end  of  a  match 
slightly  moistened. 


108  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


The  Separation  of  Minerals  by  Chemical  Means. 

The  chemical  methods  which  may  be  used  for  the  separation  of 
rock  constituents  are  so  manifold,  and  vary  so  for  particular  cases,  that 
a  general  scheme  for  their  application  cannot  be  formulated.  An  ex- 
perienced chemist  will  easily  combine  them  himself,  and  an  inex- 
perienced one  cannot  employ  them  with  success.  Without  dwelling 
upon  the  separation  of  the  carbonates  by  weak  acids,  and  of  the  well-- 
known methods  of  partial  silicate  analysis,  attention  should  be  called 
to  the  far-reaching  application  of  hydrofluoric  acid,  either  alone  or  in 
combination  with  hydrochloric  or  sulphuric  acid.  If  into  a  platinum 
dish  containing  pure  concentrated  hydrofluoric  acid  the  powder  of  a 
rock  is  gradually  introduced,  not  too  rapidly  lest  a  strong  boiling  be 
produced,  but  rapidly  enough  to  produce  a  sufficient  elevation  of 
temperature,  then  the  constituents  of  the  rock  will  be  attacked  in  a 
certain  succession — tirst  the  glassy  portions,  then  the  feldspars  and  re- 
lated substances,  then  the  quartz,  finally  the  constituents  rich  in  mag- 
nesia and  iron  belonging  to  the  pyroxene,  amphibole,  olivine,  and 
kindred  families.  Consequently,  if  the  process  is  interrupted  at  the 
proper  time  by  suddenly  adding  water,  it  is  possible  after  some  prac- 
tice to  decompose  certain  substances  and  retain  others  unattacked.  In 
this  way  the  microlites  of  glasses,  the  feldspars  of  porphyritic  ground- 
masses,  or  the  older,  more  basic  secretions  may  be  isolated,  often  com- 
pletely retaining  their  crystal  form.  During  the  action  of  the  acid 
the  powder  is  stirred  with  a  platinum  rod,  which  treatment  is  continued 
during  the  addition  of  the  water  in  order  to  break  up  the  lumps  of  gelat- 
inous silica,  and  aid  their  removal  by  the  water.  As  soon  as  it  may  be 
done  without  danger,  the  fingers  should  be  used  to  rub  the  unattacked 
crystal  powder  against  the  sides  of  the  dish  in  order  to  free  it  from  the 
gelatinous  film.  Finally,  the  water  is  drained  off  and  the  crystal 
powder  is  carefully  heated  to  redness,  by  which  means  the  gelatinous 
silica  still  adhering  is  converted  into  pulverulent  silica,  which  can  be 
easily  and  completely  washed  away.  By  proper  treatment  the  small 
crystals  of  the  .magnesia  and  iron  silicates  retain  brilliant  crystal 
faces. 

Sauer,*  Cossa,f  and  Cathrein  J  have  shown  how  a  mixture  of  hydro- 
fluoric and  hydrochloric  or  sulphuric  acids  can  be  employed  to  isolate 
rutile  from  slates.  The  same  method  permits  the  separation  of  zircon, 
tourmaline,  spinel,  andalusite,  disthene,  etc.,  from  other  silicates. 


*  N.  J.  B.  1879,  571  ;  1880,  I.  280.     f  N.  J.  B.  1880, 1.  162-164.     \  K  J.  B.  1881, 1.  172. 


SPECIFIC  GRAVITY. 


Determination  of  the  Specific  Gravity  of  the  Isolated  Powder. 

If  the  specific  gravity  of  a  powder  which  has  been  obtained  by  sep- 
aration was  not  already  discovered  during  the  separation,  and  if  the 
picnometric  determination  cannot  be  made,  it  may  be  found  by  bring- 
ing a  grain  into  suspension  in  a  separating  fluid  and  then  ascertaining 
the  density  of  the  fluid  by  means  of  Westphal's  balance.  If  one  does 
not  possess  such  a  balance,  then  the  adjusted  fluid  is  poured  from 
the  powder  into  a  calibrated  and  weighed  flask,  holding  20-25  ccm., 
filled  exactly  to  the  mark  and  weighed.  The  weight  divided  by  the 
volume  gives  the  specific  gravity.  Care  should  be  taken  in  the  first 
process  that  no  air-bubbles  are  attached  to  the  sinker  of  the  balance, 
and  in  the  second  the  determination  should  be  repeated  three  times. 
In  both  cases  the  work  should  be  done  as  quickly  as  possible  because 
of  the  hygroscopic  nature  of  the  fluid.  The  first  process  is  the  shorter  ; 
the  second  is  the  more  exact. 

The  specific  gravity  of  bodies  also  whose  density  is  considerably 
greater  than  that  of  the  separating  fluid  can  be  determined  by  means 
of  such  fluids  when  their  absolute  weight  is  not  too  small  (greater  than 
0.01  gr.).  A  small  sphere  of  wax  is  weighted  by  a  mineral  grain  en- 
closed in  it,  and  its  weight  determined  =  g.  To  this  is  attached  the 
grains  whose  specific  gravity  =  d'  is  to  be  measured,  and  whose  abso- 
lute weight  =  g'  has  been  determined.  The  system  is  now  placed 
in  the  separating  fluid,  which  is  adjusted,  and  its  density  =  D  deter- 
mined. The  wax  sphere  is  removed  from  the  solution,  the  loosely 
attached  grains  are  carefully  removed,  the  sphere  replaced  in  the  fluid, 
and  its  specific  gravity  =  d  determined.  From  this  its  volume, 

*  v  —  -,  is  known,  and  we  have  the  equation 


therefore 

g'D 


d'  = 


g+g'  -  Dv 


The  following  table  presents  the  most  important  rock-making  min- 
erals arranged  according  to  their  specific  gravities.  The  numbers 
given  signify  only  average  values,  and  may  prove  to  be  somewhat  inex- 
act in  particular  cases. 


110 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


Cassiterite      .     . 
Hematite        .     . 
Magnetite      .     . 
Ilmenite         .     . 
Chromite 
Zircon            .     . 
Rutile             .     . 

.     6.84 
.     5.30 
.     5.20 
.    4.75 
.     4.46 
.     4.45 
4.25 

Olivine      .     . 
Vesuvianite    . 
Hypersthene 
Epidote 
Zoisite 
Diopside 
Axinite 

Melauite        .     . 
Brookite        .     . 
Picotite          .     . 
Perofskite      .     . 
Corundum 
Hercynite      .    . 
Auatase     .     ;    . 
Pleonast    .     .     . 
Pyrope      .     .     . 
Staurolite  .     .     . 
Allochroito     .     . 

.     4.15 
.     4.14 
.     4.08 
.    4.06 
.     3.95 
.     3.94 
.     3.90 
.     3.82 
.     3.75 
.     3.74 
.     3.70 
3.60 

Ottrelite 
Sillinianite    . 
Hornblende    . 
Andalusite    . 
Bronzite    . 
Fluorite     .     . 
Anthophyllite 
Apatite 
Spodumene    . 
Glaucophane 
Actinolite 
Biotite       .     . 

Disthene     .     .     . 
Topaz    .... 

.     3.60 
3.56 

Gehlenite 
Prehnite    .     . 

Ouvarovite     .     . 
Grossular  .     .     . 
Acrnite       .     .     . 
Titanite     .     .     . 
Arfvedsonite 

.     3.51 
.     3.50 
.     3.49 
.     3.48 
.     3.45 

Melilite      .     . 
Dolomite  . 
Wollastonite 
Muscovite 
Chlorite     .     . 

3.41 
3.40 
3.39 
3.39 
3.35 
3.30 
3.29 
3.26 
3.23 
3.22 
3.20 
3.19 
3.18 
3.17 
3.16 
3.14 
3.10 
3.02 
3.01 
2.95 
2.94 
2.93 
2.90 
2.86 
2.85 
2.78 


Anorthite      ....  2.76 

Lazulite 2.75 

Talc 2.74 

Meionite  ...          .2.73 

Beryl 2.72 

Bastite      .     .     .     .     .  2.70 

Dipyre 2.66 

Quartz      ,     .   -.    .     .  2.65 

Albite  ......  2.63 

Elseolite 2.60 

Sanidiue 2.56 

Nepheline     ....  2.55 

Leucite 2.47 

Cancrinite    .     .     .     .  2.46 

Haiiyne 2.45 

Petalite 2.39 

Brucite 2.36 

Gypsum 2.31 

Sodalite 2.28 

Natrolite       .     .     .     .  223 

Opal 2.21 

Analcite 2.19 

Hyalite 2.17 

Chabasite               .  2.10 


Determination  of  the  Hardness  of  the  Isolated  Powder. 

The  grains  of  powder  are  pressed  firmly  into  the  smoothly  filed  end 
of  a  leaden  stamp  a  few  millimetres  thick,  which  is  used  as  a  handle  in 
carrying  out  experiments  in  scratching  the  faces  of  minerals  of  known 
hardness.  Approximate  determinations  may  be  made  by  rubbing  the 
powder  between  two  object-glasses  and  observing  whether  these  are 
scratched  or  not.  In  the  latter  case  the  grating  and  the  easier  or 
harder  crushing  of  the  grains  between  the  glasses  indicates  the  hard- 
ness. 

Chemical  Reactions. 

These  are  the  same  whether  carried  on  upon  isolated  powder  or 
directly  on  the  thin  section.  It  is  first  necessary  to  bring  the  sub- 
stance to  be  investigated  into  solution.  For  non-silicates  this  is 
done  by  the  well-known  methods.  Silicates  are  decomposed  by 
direct  treatment  with  hydrofluosilicic  acid  or  with  hydrofluoric  acid. 
It  is  always  preferable  to  use  the  substance  to  be  investigated  in  the 
form  of  isolated  powder.  The  results  of  the  hydrofluosilicic-acid 
method  have  been  carefully  worked  out  and  described  by  Boricky  ; 
those  derived  by  using  hydrofluoric  and  sulphuric  acids,  by  Behrens 
and  others. 

Boricky's  method  of  treating  the  silicates  with  hydrofluosilicic  acid 


CHEMICAL  REACTIONS.  Ill 

is  ns  follows  :  An  object-glass  is  covered  with  a  thin,  even  coat  of 
Canada  balsam  ;  on  this  is  placed  one  or  more  grains  of  the  mineral 
about  the  size  of  a  poppy-seed.  These  may  be  fastened  to  the  balsam 
and  thus  kept  in  place  by  gently  heating  the  object-glass.  The  grains 
are  covered  with  a  spherical  drop  of  hydrofluosilicic  acid,  which  shonld 
not  be  allowed  to  spread  over  the  balsam.  The  balsam  should  not  be 
•cracked,  as  the  acid  would  attack  the  glass.  The  hydrofluosilicic  acid 
must  be  absolutely  pure,  and  should  leave  no  residue  upon  being 
evaporated.  The  mineral  fragment  to  be  investigated  must  be  dis- 
solved as  completely  as  possible  ;  for  otherwise  the  crystallizations 
formed  upon  the  drying  up  of  the  acid  would  give  a  false  idea  of  the 
Composition  of  the  substance.  The  evaporation  of  the  solution,  which 
is  naturally  very  slow,  may  be  accelerated  and  the  action  of  the  reagent 
advanced  by  gently  warming  the  object-glass  over  a  spirit-lamp. 
Upon  the  drying  of  the  solution  there  arise  characteristic  crystalliza- 
tions in  the  form  of  fluosilicates  of  the  univalent  and  bivalent  elements 
which  were  present  in  the  mineral  investigated.  The  fluosilicate  of 
aluminium  is  gelatinous.  If  the  crystallization  is  incomplete  in  con- 
sequence of  too  rapid  evaporation,  it  should  be  redissolved  in  water 
or  in  a  drop  of  dilute  hydrofluosilicic  acid,  transferred  to  a  new 
object-glass,  and  allowed  to  crystallize  anew.  If  the  mineral  was  not 
completely  dissolved,  it  is  treated  again  with  a  fresh  drop  of  hydrofluo- 
silicic acid.  Many  silicates,  especially  mica,  cannot  be  completely  de- 
composed even  by  concentrated  hydrofluosilicic  acid  ;  these  are  then 
decomposed  by  hydrofluoric  acid  in  a  small  platinum  dish,  evaporated 
to  dry  ness  after  the  addition  of  an  excess  of  hydrofluosilicic  acid,  taken 
up  with  distilled  water,  and  the  solution  evaporated  on  an  object-glass. 
In  studying  these  extremely  minute  crystallization  products  it  is  best 
to  improve  the  illumination  by  lowering  the  polarizer. 

The  reactions  and  crystallizations  most  frequently  used  in  the 
diagnosis  of  rock-making  minerals  are  the  following : 

Potassium. — Upon  the  drying  of  the  hydrofluosilicic  compound 
there  form  isotropic,  colorless  crystals  of  K2SiFl6,  in  cubes,  octahe- 
drons, or  combinations  of  these  forms  with  the  rhombic  dodecahedron 
(PL  XII.  Figs.  1  and  2).  From  a  very  acid  solution  and  at  a  low  tempera- 
ture there  sometimes  arise  anisotropic  crystals  of  apparently  orthorhom- 
bic  form,  especially  when  the  solution  contains  a  high  percentage  of 
sodium.  If  these  are  dissolved  in  hot  water  and  again  allowed  to  crys- 
tallize out,  they  assume  the  normal  forms.  In  hydrochloric-acid  or 
sulphuric-acid  solutions  after  the  liberation  of  the  potassium  with 
hydrofluoric  acid  there  are  formed  with  platinic  chloride  sharply  devel- 


112  PHYSIOGRAPHY  OF  THE  ROCK- MAKING  MINERALS. 

oped   yellow  octahedrons   of   potassium    platinic   chloride  (K2PtCl6% 
more  rarely  cubes  or  crystals  rich  in  combinations. 

Sodium. — From  the  hydrofluosilicic-acid  solution  there  arise  upon 
evaporation  crystals  of  Na2SiFl6  in  hexagonal  combinations  (PI.  XI. 
Figs.  4-6)  oo  P  .  oP  (1010)  (0001)  or  oo  P  .  P  (1010)  (1011).  They 
lie  sometimes  on  the  prism  faces,  sometimes  on  the  base  ;  they  are 
colorless,  very  weakly  doubly  refracting,  with  negative  character.  The 
pyramid  is  quite  obtuse  (1011)  :  (1011)  =  66°  6'.  The  crystals  are 
generally  longer  the  more  calcium  there  is  in  the  solution.  This  test 
is  exceedingly  sharp  and  certain  even  for  very  snuill  amounts. 

Calcium. — Upon  the  evaporation  of  the  hydrofluosilicic-acid  solu- 
tion there  form  monoclinic  crystals  of  CaSiFJ.  +  2aq,  which  assume 
a  great  variety  of  forms.  Sometimes  they  are  in  pointed,  thorn-like 
or  ramified  groups  and  single  crystals,  sometimes  in  rhomboid  plates, 
most  frequently  in  spindle-shaped  individuals,  with  not  very  strong 
double  refraction  (PI.  XII.  Figs.  4:  and  5j.  It  is  very  characteristic  of  all 
the  forms  that  they  seldom  have  straight-edged  boundaries,  but  gener- 
ally crooked  ones.  Upon  the  addition  of  dilute  sulphuric  acid  they 
are  decomposed,  and  there  is  deposited  in  their  place  long  prismatic 
crystals  of  gypsum.  Upon  decomposition  with  hydrofluoric  and  sul- 
phuric acids  only  a  part  of  the  calcium  sulphate  goes  into  solution,  if 
the  percentage  of  calcium  in  the  silicate  is  large ;  with  a  smaller  per- 
centage of  calcium  and  an  excess  of  dilute  sulphuric  acid,  all  of  the 
calcium  goes  into  solution  without  residue,  and  there  forms  about  the 
edge  of  the  drop  the  characteristic  prisms  and  plates  of  gypsum  ooP . 
ooPcb.P  (110)  (010)  (111),  usually  lying  on  (010)  (PI.  XIII.  Fig. 
1).  This  is  the  most  delicate  and  the  surest  test  for  Ca. 

Magnesium. — From  the  hydrofluosilicic-acid  solution  there  crystal- 
lizes MgSiFl6  +  6aq  in  rhombohedral  crystals,  which  most  frequently 
exhibit  the  combination  ooP2 .  7?,  (1120)  7r(1011),  more  rarely  E .  coP2 
or  It .  oR.  They  are  always  very  sharp-edged,  and  have  plane  faces ; 
are  strongly  doubly  refracting,  with  positive  character ;  generally  polar- 
ize in  bright  colors  of  the  second  order,  and  exhibit  when  in  the  proper 
position  a  very  distinct  interference  figure.  They  are  colorless  (PL 
XII.  Fig.  6).  The  formation  of  Stru  vite  crystals  (NH4MgPO4  +  6aq), 
with  their  coffin-like,  hemimorphic  forms,  is  a  very  characteristic  reac- 
tion (PI.  XIII.  Figs.  3  and  4).  The  crystals  may  be  obtained  most  per- 
fectly from  a  very  dilute  solution,  to  which  ammonium  chloride  and 
ammonia  is  added  to  distinctly  alkaline  reaction.  A  grain  of  salt  of 
phosphorus  is  placed  at  the  edge  of  the  solution,  or  a  drop  of  sodium 
phosphate  is  added  to  it.  The  crystals  separate  slowly  in  the  cold 


CHEMICAL  REACTIONS.  113 

solution,  rapidly  in  a  warm  one.  But  in  the  latter  case  there  arise 
forms  of  growth  very  difficult  to  recognize ;  these  also  form  at  first 
in  concentrated  solutions,  and  it  is  only  after  the  greater  part  of  the 
salt  has  separated  out  that  the  characteristic  crystals  begin  to  form. 

Iron. — Upon  the  evaporation  of  the  hydrofluosilicic-acid  solution 
crystals  of  FeSiFl6  -f-  6aq  are  deposited,  which  are  completely  isomor- 
phous  with  those  of  the  magnesium  salt,  and  have  the  same  optical 
characters.  They  may  be  distinguished  from  the  latter  by  being 
moistened  with  potassium  ferrocyanide  or  with  ammonium  sulphide: 
in  the  first  case  they  become  blue ;  in  the  second,  black.  The  amor- 
phous precipitate  with  potassium  ferrocyanide  or  with  ammonia  is  also 
very  easily  recognized. 

Aluminium. — This  separates  out  of  a  hydrofluosilicic-acid  solu- 
tion in  a  gelatinous  state.  From  a  sulphuric-acid  solution  after  the 
addition  of  a  slight  amount  of  caesium  chloride  or  of  caesium  sulphate, 
there  are  deposited  isometric  crystals  of  caesium  alum.  The  pre- 
dominant forms  of  these  are  0,  0.  oc#oo  ;  the  less  frequent  form  is 
oo  0  GO,  which  usually  separates  from  neutral  solutions.  The  crystals 
apparently  never  exhibit  the  optical  anomalies  so  common  to  the 
alums.  If  the  solution  is  too  concentrated,  there  arise  many  branched 
forms  of  growth  ;  these  may  be  dissolved  in  water  and  again  allowed 
to  crystallize.  Too  great  an  excess  of  sulphuric  acid  retards  the  for- 
mation of  crystals;  this  is  best  neutralized  by  the  addition  of  sodium 
acetate  (PI.  XIII.  Fig.  2). 

Chlorine. — This  test  is  important  for  the  minerals  of  the  sodalite 
group.  The  powdered  substance  is  put  in  a  small  hemispherical 
platinum  dish  and  covered  with  somewhat  concentrated  sulphuric 
acid.  The  dish  is  covered  with  a  small  glass  cover,  on  the  under  sur- 
face of  which  is  a  drop  of  water,  while  on  the  upper  surface  another 
drop  of  water  serves  to  keep  it  cool.  The  dish  is  warmed  moderately, 
and  the  escaping  chlorine  is  caught  in  the  drop  on  the  glass  cover. 
After  the  upper  drop  has  been  removed  the  glass  cover  is  taken  off, 
and  the  drop  beneath  it  containing  the  distillation  is  placed  on  an 
object-glass.  If  a  grain  of  thallium  sulphate  is  put  in  it  there  form 
octahedrons,  and  the  combinations  of  0 .  oo  0  of  thallium  chloride. 
These  are  strongly  refracting,  and  with  low  magnify  ing-powers  are 
almost  opaque  in  consequence  of  the  total  reflection.  Or  by  adding 
silver  nitrate  flocculent  silver  chloride  may  be  obtained,  to  which  if 
strong  ammonia  be  added  and  the  whole  dried,  isometric  crystals 
(O  and  GO  0  oo,  rarely  with  co  0)  of  silver  chloride  are  formed,  which 
are  strongly  refracting. 


114          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

Sulphur  is  tested  as  sulphuric  acid ;  upon  adding  to  the  solution 
containing  it  a  calcium  salt  the  characteristic  gypsum  crystals  are 
produced. 

Phosphorus  only  occurs  in  the  phosphates  among  the  rock-making 
minerals.  The  soluble  phosphates  may  be  treated  with  the  nitric-acid 
solution  of  ammonium  molybdate ;  the  solution  when  dried  gives 
rhombic  dodecahedral  crystals  of  the  well-known  precipitate  (PI.  XIII. 
Fig.  5),  which  are  yellow  by  incident  light  and  green  in  transmitted 
light.  If  the  mineral  substance  tested  is  not  pure,  and  there  be  any 
soluble  silica  present,  it  may  be  rendered  insoluble  by  being  dried  on 
an  object-glass ;  the  residue  is  again  dissolved  in  nitric  acid  and  the 
reagent  added.  Insoluble  phosphates  are  first  decomposed  with  soda. 
The  reaction  with  ammonium  chloride  and  magnesium  sulphate  is  just 
as  sharp ;  by  this  method  the  phosphoric  acid  is  obtained  as  crystals  of 
ammonium  magnesium  phosphate  (PI.  XIII.  Figs.  3  and  4). 

Titanium. — If  a  titaniferous  mineral  be  carefully  melted  on  plati- 
num wire  with  a  grain  of  potassium  bi sulphate,  and  then  placed  in  a 
porcelain  dish  and  moistened  with  a  drop  of  hydrogen  superoxide  in 
water,  the  bead  and  the  solution  will  be  colored  yellow  or  orange-yel- 
low, according  to  the  amount  of  titanic  acid  present,*  The  reaction  is 
extremely  sharp,  even  for  the  smallest  amounts. 

*  Schoner,  Zeitschr.  f.  analyt.  Cheroje,  1870,  IX.  41. 


115 


SPECIAL  PART. 


SYSTEM    OF  CLASSIFICATION. 

THE  determination  of  minerals  under  the  microscope  is  based  prim- 
arily on  the  investigation  of  their  optical  properties  in  connection 
with  their  crystal  forms,  as  these  are  indicated  by  outline  and  cleavage 
(Blatterdurchgange).  Hence  the  minerals  treated  in  the  descriptive 
part  of  this  book  are  arranged  according  to  their  system  of  crystal- 
lization. From  the  introductory  or  general  part  it  is  evident  that 
mineral  bodies  may  be  classified  according  to  their  optical  and  crystal- 
lographic  characters  in  the  following  groups  : 

1.  Isotropic  minerals  ..................................  j  fsZetric""  SUbStanCes' 

f  a.  With  one  optic  axis    f  Tetragonal. 
(optically  uniaxial).    \  Hexagonal. 

3   AniS°troPiCminmlS  .......  '  ......  ]  ».  With  two  optic  axes 

(optical* 


Besides  these  there  is  a  small  number  of  cryptocrystalline  substances, 
which  are  definitely  characterized  as  such  by  their  double  refraction, 
bufc  which  always  occur  in  such  imperfect  forms  and  in  such  micro- 
scopical aggregation  that  their  crystal  system  cannot  be  determined 
with  certainty.  These  will  be  placed  under  the  head  of  aggregates. 

The  method  of  procedure  which  should  be  adopted  in  a  micro- 
scopical determination  will  be  briefly  given.  The  question  which  first 
arises  is  whether  the  substance  is  optically  a  unit  (single  individual), 
or  an  aggregate,  the  crystal  system  of  whose  component  individuals 
•cannot  be  determined  by  their  form  nor  by  their  behavior  toward  pol- 
arized light.  A  substance  is  recognized  as  a  unit  or  individual  by  the 
fact  that  it  shows  the  same  optical  behavior  throughout  its  whole 
extent,  so  far  as  this  is  not  modified  by  twinning.  The  substance  is 
therefore  first  studied  in  parallel  light  between  crossed  nicols  to  see 
whether  the  extinction  of  the  light  takes  place  at  one  and  the  same 
time  throughout  the  whole  extent  of  the  substance,  or  of  the  parts  of 
the  twins,  when  it  is  rotated  with  the  stage  of  the  microscope.  If  this 


116          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

is  not  the  case,  but  if  the  substance  which  appeared  as  a  unit  in  ordi- 
nary light  is  separated  into  a  number  of  individuals,  which  are  irregu- 
larly bounded,  or  are  combined  in  a  fibrous  or  laminated  spherical 
form  and  are  not  separately  determinable,  and  for  which,  collectively, 
the  extinction  is  never  synchronous  during  a  rotation  between  crossed 
nicols,  then  the  substance  is  an  aggregate  (cf.  p.  88). 

If  the  substance  is  found  to  be  optically  a  unit,  it  is  then  necessary 
to  observe  whether  it  belongs  to  the  isotropic  or  to  the  anisotropic 
division.  An  isotropic  substance,  in  which  the  elasticity  of  the  lumi- 
niferous  ether  is  the  same  in  all  directions  is  characterized  (p.  52)  by 
the- fact  that  it  remains  dark  in  parallel  light  between  crossed  nicols 
during  a  complete  rotation,  while  it  shows  the  same  color  and  inten- 
sity of  light  in  all  positions  between  parallel  nicols.  An  isotropic  sub- 
stance can  never  show  interference  colors  whatever  may  be  its  posi- 
tion or  the  direction  of  the  nicols  to  one  another.  Since,  now,  doubly 
refracting,  uniaxial  bodies  may  behave  in  the  same  manner  when  they 
are  intersected  at  right  angles  to  an  optic  axis,  it  is  necessary,  after 
establishing  the  isotropic  behavior  of  a  substance  in  parallel  light,  to 
test  it  in  convergent  light.  If  the  interference  figure  of  a  uniaxial 
body  is  not  obtained  by  this  latter  test,  the  substance  is  isotropic,  and 
is  either  amorphous  or  of  the  isometric  system.  In  order  not  to  over- 
look a  very  slight  double  refraction  which  may  lead  to  confusion  when 
the  investigation  is  only  made  between  crossed  nicols,  and  the  field  of 
view  is  not  completely  dark,  but  is  somewhat  gray,  one  of  the  stauro- 
scopic  methods  of  testing  in  parallel  light  described  on  page  63  should 
be  used. 

Whether  an  isotropic  substance  is  amorphous  or  is  crystallized  in 
the  isometric  system  is,  in  general,  easily  determined  by  the  form  of 
the  body,  and  by  the  presence  or  absence  of  cleavage.  Amorphous 
substances  exhibit  no  independent  boundary,  their  form  always  depend- 
ing on  those  of  the  surrounding  substances ;  they  do  not  possess  cleav- 
age ;  and  the  lines  of  internal  parting  when  present  are  not  straight. 

If  the  substance  under  investigation  is  anisotropic,  it  is  next  neces- 
sary to  find  whether  it  belongs  to  the  optically  uniaxial  division  or  to 
the  optically  biaxial.  In  order  to  do  this,  those  crossed  sections  are 
sought  which  show  the  least  difference  of  color  during  rotation  in 
parallel  light  between  crossed  nicols,  or,  when  possible,  such  as  re- 
main dark  or  uniformly  clear  for  all  positions  during  rotation.  The 
first  will  show  the  axial  cross  of  uniaxial  substances  (page  68)  in  con- 
vergent light;  the  second,  the  axial  bar  of  biaxial  substances  (page 
72)  with  or  without  isochromatic  curves.  All  other  sections  will 


SYSTEM  OF  CLASSIFICATION.  117 

exhibit  a  maximum  extinction  four  times  during  a  complete  rotation 
between  crossed  nicols,  which  maxima  are  90°  apart ;  and  in  the  in- 
termediate positions  they  will  show  an  interference  color  whose  maxi- 
mum lies  exactly  in  the  middle  between  the  directions  of  maximum 
extinction.  If  the  substance  is  found  to  be  optically  uniaxial,  the  dis- 
tinction between  the  tetragonal  and  hexagonal  systems  is  determined 
by  the  cleavage  and  the  crystal  form.  Isotropic  sections  of  tetragonal 
substances  which  show  axial  figures  in  convergent  light  are  bounded 
by  quadratic  or  octagonal  outlines,  or  show  two  cleavages  at  right 
angles  to  one  another.  For  hexagonal  substances  these  outlines  are 
hexagonal,  trigonal,  or  nine-sided,  and  the  cleavage  forms  equilateral 
hexagons  or  triangles.  These  sections  of  tetragonal  or  hexagonal  min- 
erals which  are  not  isotropic  show  very  different  outlines  according  to 
the  crystal  form  of  the  substance.  However,  all  sections  in  the  prism- 
zone  generally  exhibit  parallel  edges  and  parallel  cleavage,  that  is,  bi- 
symmetric  outlines  and  cleavage  lines  respectively,  and  the  extinction 
will  always  take  place  when  their  directions  of  symmetry  lie  parallel 
to  the  principal  sections  of  the  nicols  In  sections  in  other  zones, 
also,  the  directions  of  extinction  lie  parallel  or  at  right  angles  to  the 
directions  of  symmetry,  whenever  the  sections  possess  any. 

If  the  substance  is  anisotropic  and  optically  biaxial,  it  remains  to 
be  found  whether  the  crystal  system  is  orthorhombic,  monoclinic,  or 
triclinic.  This  may  be  done  in  many  cases  by  investigating  the  axial 
•dispersion  in  sections  which  lie  at  right  angles  to  the  acute  bisectrix. 
From  the  nature  of  things  these  sections  are  rare,  consequently  the 
position  of  the  axes  of  elasticity  must  be  investigated. 

In  the  orthorhombic  system  these  axes  lie  parallel  to  the  crys- 
tal axes.  Hence  all  sections  in  a  zone  oP  :  oo  P  66,  oP  :  oo  P  db, 
oo  P  60  :  oo  Pob  will  extinguish  parallel  to  the  axis  of  the  zone. 
This  zonal  axis  is  easily  recognized  by  the  form  of  the  outlines  which 
are  bisymmetric  or  monosymmetric,  or  by  the  cleavage.  The  cleavage 
in  the  orthorhombic  rock-making  minerals  is  pinacoidal  or  prismatic, 
and  the  extinction  lies  parallel  and  at  right  angles  to  the  cleavage 
cracks  when  these  are  parallel,  and  bisects  the  angle  between  them 
when  they  intersect  one  another.  In  sections  which  do  not  belong  to 
one  of  the  principal  zones  just  named,  the  extinction  lies  parallel  and 
at  right  angles  to  the  directions  of  symmetry  of  the  outline,  or  of  the 
cleavage  cracks,  whenever  they  are  present.  Stauroscopic  methods 
must  often  be  employed  for  the  exact  determination  of  the  directions 
of  extinction.  If  the  sections  in  the  three  principal  zones  are  tested 
in  convergent  light,  some  will  be  found  exhibiting  the  point  of  emerg- 


118          PHY8IOGEAPHT  OF  THE  ROCK-MAKING  MINERALS. 

ence  of  a  bisectrix ;  and  when  these  are  observed  on  cleavage  plates 
the  bisectrices  will  emerge  perpendicularly  (for  pinacoidal  cleavage),  or 
will  be  inclined  to  one  side  (for  prismatic  cleavage). 

In  the  monoclinic  system  only  one  axis  of  elasticity  coincides  with 
a  crystal  axis,  which  is  always  the  axis  5.  The  outline  and  cleavage 
can  only  form  symmetrical  figures  in  the  zone  oP  :  oo  P  6b  (001  :  100), 
and  in  these  the  extinction  lies  parallel  to  the  directions  of  symmetry, 
that  is,  parallel  and  perpendicular  to  the  axis  b.  In  the  vertical  zones 
as  well  as  in  the  zone  oPj.  oo  P  do  (001  :  010),  the  extinction  no  longer 
lies  parallel  to  the  outKifes  or  to  the  cleavages,  which  in  this  system 
run  parallel  (oP,  oo  P  60)  to  the  axis  5  or  in  the  projection,  at  least, 
perpendicular  (oo  .P  oo,  oo  P)  to  this  axis.  This  inclined  extinction  is- 
an  important  characteristic.  Cleavage  plates  of  monoclinic  minerals 
parallel  to  the  pinacoids  can  only  show  a  bisectrix  perpendicular  to 
their  planes  when  the  pinacoid  is  the  plane  of  symmetry ;  the  disper- 
sion is  then  crossed  :  on  the  other  pinacoids  a  bisectrix  either  does  not 
appear  at  all,  ^Jemerges  at  one  side  of  the  field  with  considerable  in- 
clination. 

In  the  triclinic  system  all  sections  are  asymmetric.  In  general  the 
direction  of  extinction  lies  oblique  to  the  outline  or  to  the  cleavage 
lines. 

These  statements  may  be  tabulated  as  follows : 

I.  The  substance  has  the  same  optical  orienta- 
tation  throughout  its  whole  extent,  or  the 
parts  having  different  optical  orientation 
are  bounded  rectilinearly  (twinned).  HOMOGENEOUS  and  UNIFORM. 

(1)  All  sections  of  the  same  substance  re- 

main dark  during  a  complete  rota- 
tion in  parallel  light  between  crossed 
nicols,  and  give  no  interference  fig- 
ure in  convergent  light.  ISOTROPIC. 
(la)  All     independent     form    and 

cleavage  is  wanting.  Amorphous. 

(16)  Independent  form  or  cleavage 

is  present.  Isometric. 

(2)  The  sections  generally  show  an  inter- 

ference figure  between  crossed  nicols 
and  become  dark  four  times  during 
a  rotation  between  crossed  nicols. 
The  sections,  which  remain  dark,  or 
nearly  so,  during  a  complete  rota- 
tion between  crossed  nicols,  show  a 
dark  cross  in  convergent  light,  with 
or  without  isochrornatic  circles,  the 
arms  of  the  cross  remaining  un- 


SYSTEM  OF    CLASSIFICATION. 


119 


changed  or  moving  parallel  to  them- 
selves during  the  rotation. 
(2a)  The  sections  showing  uniaxial 
interference  figures  are  quad- 
ratic (octagonal)  or  show  rec- 
tangular cleavage. 
(2b)  The  sections  showing  uniaxial 
interference  figures  are  hex- 
agonal, trigonal  or  nine-sided, 
or  show  systems  of  cleavage 
cracks  intersecting  at  60°. 
(3)  The  sections  in  general  show  an  inter- 
ference color  between  crossed  nicols 
and  become  dark  four  times  during 
a  complete  rotation.      No  section 
remains  dark  during  a  whole  rota- 
tion.    Those   sections  showing  no 
interference  color,  but  having  a  uni- 
form illumination  for  all  positions, 
show  the  locus  of  an  optic  axis. 
(3a)  Sections  in  the  three  principal 
zones  are  symmetrical  with 
respect  to  outline  and  cleav- 
age, and  the  extinction  lies 
parallel  and  perpendicular  to 
the  directions  of  symmetry. 
(36)  The    outline   and    cleavage  is 
only  symmetrical  in  the  zone 
oP :  oo  Poo,   and  this  is    the 
only  zone  in  which  the  ex- 
tinction lies  parallel  and  per- 
pendicular to  the  directions 
of  symmetry. 

(3c)  The  outline,  cleavage,  and  ex- 
tinction are  unsymmetrical  in 
all  zones. 

II.  Different  parts  of  the  same  substance  show 
different  optical  behavior,  and  the  indi- 
vidual parts  are  not  regularly  bounded 
nor  regularly  optically  oriented. 


ANISOTROPIC,  OPTICALLY  UNIAXIAL. 


Tetragonal. 


Hexagonal. 


ANISOTROPIC,  OPTICALLY  BIAXIAL. 


Orthorhombic. 


Monoclinic. 
Triclinic. 

AGGREGATE. 


The  crystal  system  of  the  few  minerals  which  remain  opaque  in 
thin  section  must  necessarily  be  determined  by  their  outline  and  cleav- 
age alone.  There  remains,  however,  a  considerable  number  of  com- 
pletely transparent  minerals  whose  optical  behavior  in  part  is  in  evi- 
dent contradiction  to  their  crystallization  (optically  anomalous  min- 
erals), or  whose  optical  behavior  in  certain  instances  strikingly  ap- 
proaches that  of  a  crystal  system  other  than  that  to  which  they  belong. 
Optically  anomalous  bodies  are  frequent  among  the  isometric  minerals 


120          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

as  well  as  among  the  tetragonal  and  hexagonal ;  many  garnets,  perof- 
skite,  leucite,  apophyllite,  and  tridymite  are  familiar  examples.  Min- 
erals of  the  mica  family  are  among  the  substances  which  in  many  in- 
stances show  an  optical  behavior  approaching  that  of  another  crystal 
system  ;  from  their  action  in  polarized  light  these  have  in  some  cases 
nearly  a  uniaxial  character,  in  others  nearly  an  orthorhombic  character. 
Of  triclinic  minerals  oligoclase  and  andesine  show  an  approximately 
monoclinic  orientation  of  their  direction  of  extinction  in  the  zone  of 
their  principal  cleavage. 


OPAL.  121 


AMOEPHOUS  MINERALS. 

AMORPHOUS  minerals 'are  produced  by  the  chilling  of  melted  bodies 
or  by  the  solidification  of  gelatinous  ones.  The  first  may  be  called 
glasses,  the  second  opals  (hyaline  and  porodine  substances). 

The  glasses  occur  either  as  independent  geological  bodies,  in  which 
case  they  are  considered  with  the  rocks,  as  obsidian,  pearlite,  pumice, 
pitchstone,  tachylite,  hyalomelan,  palagonite,  sideromelane,  sordawalite, 
and  wichtisite;  or  they  appear  as  the  residuum  of  crystallization  in 
certain  porphyritic  rocks.  In  the  latter  case,  since  their  composition 
varies  greatly  according  to  the  manner  and  extent  to  which  crystalliza- 
tion has  advanced  previous  to  their  consolidation,  they  cannot  be 
treated  as  minerals,  but  will  be  described  under  those  rock  groups  in 
which  they  are  found. 

Of  the  little-known  porodine  amorphous  minerals  only  the  differ- 
ent varieties  of  amorphous  silica  have  a  general  distribution  among 
rocks — in  most  cases  as  products  of  the  leaching  out  and  of  the  altera- 
tion of  silicates;  in  others  it  occurs  in  the  form  of  concretionary 
masses,  where  silica  has  accumulated  through  the  action  of  organisms 
on  hydrous  solutions ;  and  to  a  very  small  extent,  if  at  all,  as  the  last 
residuum  of  crystallization  of  acid  eruptive  masses. 

OPAL. 
Literature. 

H.  BEHRENS,  Mikroskopische  Untersuchungeu  iiber  die  Opale.     Sitzber.  d.  Wien. 

Akad.  LX1V.  1871.  1.  Abthl. 

E.  REUSCH,  Uebereiuen  Hydrophan  von  Czerwenitza.  Fogg.  Ann.  CXXIV.  1865.  431. 
MAX  SCHULTZE,  Verkandlimgen   des  naturhist.  Ver.   d.   preuss.   Rheinlande  und 

Westphalens.  XVIII.  1861.  69. 
Sir  DAVID  BREWSTER,  On  the  cause  of  the  colors  in  precious  opal. — Edinb.  New 

Phil.  Journ.  by  Jameson.  XXXVIII.  1845.  385. 

Opal  is  wholly  amorphous  and  singly  refracting ;  its  varieties  are 
known  as  precious  opal,  fire-opal,  and  common  opal,  according  to  the 
color  and  to  the  presence  or  absence  of  iridescence.  Opal  forms  irreg- 
ularly bounded  patches,  strings,  and  veins,  as  well  as  pseudomorphs 
after  feldspar,  augite,  and  other  minerals  in  decomposed  eruptive  rocks 
of  the  trachyte  and  andesite  series,  and  in  related  massive  rocks  of 
coarsely  crystalline  texture.  The  substance  of  the  opal  is  often  ren- 
dered impure  by  remnants  of  the  constituents  of  the  rocks  mentioned, 


122          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

as  well  as  by  flakes  of  hematite  or  by  very  small  and  indeterminable 
dust-like  particles.  Spots  which  are  dull  by  incident  light  and  also 
appear  cloudy  in  transmitted  light  belong  to  hydrophane  according  to 
Behrens.  They  absorb  water  eagerly,  and  become  completely  trans- 
parent, air-bubbles  escaping  during  the  process. 

When  the  opal  in  rocks  or  along  crevices  in  them  assumes  the 
spherical  form  it  often  exhibits  in  parallel  polarized  light  the  inter- 
ference'cross  of  amorphous  spherical  substances  with  centripetal  con- 
densation (p.  89). 

The  energetic  double  refraction  which  is  occasionally  shown,  espe- 
cially by  the  more  precious  varieties  of  opal,  is  occasioned  by  strains 
which  are  supposed  to  arise  from  the  unequal  drying  of  the  gelati- 
nous silica.  The  beautiful  play  of  colors  of  the  precious  opal  wras  ex- 
plained by  Brewster  by  the  presence  of  a  succession  of  cavities  whose 
varying  dimensions  gave  rise  to  different  colors.  Reusch  observed 
that  the  color  phenomena  of  precious  opal  and  hydrophane  are  com- 
plementary by  incident  and  transmitted  light,  and  explained  this  by 
the  assumption  of  cracks  which  are  parallel  to  the  plates  of  the  mineral, 
or  are  slightly  inclined  to  their  surface,  and  act  like  thin  plates. 
Behrens  could  find  no  cavities  in  the  opals  examined  by  him,  and 
explained  the  color  phenomena  as  the  result  of  thin  lamellae  of  an  opal 
of  differing  index  of  refraction,  which  may  be  inclosed  in  the  normal 
opal.  Since  the  power  of  refraction  of  opal  changes  with  the  amount 
of  water  present,  one  may  assume  that  the  variable  amount  of  water 
contained  in  the  mass  of  the  opal  gives  rise  to  the  play  of  colors.  For 
opal  Up  =  1.442  to  1.450. 

From  the  small  value  of  the  index  of  refraction  of  opal  compared 
with  that  of  Canada  balsam  it  may  be  easily  overlooked,  and  the  opal 
portion  of  a  thin  section  mistaken  for  a  cavity  filled  with  balsam. 

Colorless  hyalite  usually  forms  botryoidal  and  reniform  aggregates 
along  crevices  and  in  cavities  in  phonolitic,  tephritic.  and  basaltic 
eruptive  rocks.  np  =  1.437 -and  1.455.  Hyalite  is  doubly  refracting, 
and  under  certain  conditions  exhibits  the  interference  cross  of  a  uni- 
axial  substance  with  isochromatic  curves.  This  appearance  is  referred 
to  conditions  of  strain  occasioned  by  the  concentric  shell-like  structure 
of  the  mineral.  The  character  of  the  double  refraction  is  negative. 
The  hyalite  cross  often  separates  into  hyperbolas  during  a  rotation  of 
the  section  between  crossed  nicols,  which  would  necessarily  be  the 
case  if  the  layers  were  not  regular  spherical  shells. 

Common  opal  and  half  opal  are  distinguished  from  precious  opal 
and  hyalite  by  a  greater  admixture  of  foreign  inclusions ;  among  these 


CARBONACEOUS  MATTER.  123 

tridymite  is  of  special  interest.  G.  Rose*  first  discovered  this  mineral 
in  the  opal  of  Kosemiitz,  Silesia,  Iceland,  Kaschau,  Persia,  and  Zima- 
pan,  Mexico.  It  occurs  in  the  form  of  round  or  hexagonal  plates,  and 
in  concretions  of  these  forms. 

All  silica  hydrates  are  soluble  in  caustic  potash ;  the  presence  of 
finely  divided  opal  substance  in  clay  slates  and  sandstone  can  be  proven 
by  this  reaction.  The  specific  gravity  of  opal  varies  with  its  impurities 
between  1.9  and  2.3.  For  the  pure  varieties  sp.  gr.  =  2.2. 

Carbonaceous  Matter 

occurs  in  sedimentary  rocks  of  widely  different  formations  in  finely 
disseminated  particles,  occasionally  in  somewhat  larger  accumulations, 
or  else  in  such  a  finely  divided  state  that  it  is  only  recognized  as  the- 
coloring  of  the  other  minerals.  The  carbonaceous  flakes  are  without 
regular  boundary,  and  are  opaque,  lustreless,  gray  to  grayish  black  by 
incident  light.  They  are  unaffected  by  acids ;  when  heated  to  redness 
on  platinum  foil  they  burn  up.  They  are  often  intimately  mixed  with 
the  iron  ores  (pyrite,  limonite,  magnetite),  and  then  they  are  consumed 
with  difficulty ;  if  the  heated  section  be  treated  with  acids  and  again 
heated  to  redness,  the  dark-colored  spots  will  become  light.  It  is  not 
definitely  known  whether  these  dark  combustible  particles  are  always 
carbon  or  sometimes  a  carbon  compound.  They  color  mineral  sub- 
stances black  to  gray,  and  when  very  finely  divided,  bluish. 

*  Monatsber.  d.  Berl.  Akad.  1869.     Sitzung  vom  3.  Juni.  p.  449. 


124         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


MINERALS   OF  THE  ISOMETPJC   SYSTEM. 

MINERALS  of  the  isometric  system  differ  from  all  other  crystalline 
substances  by  the  absence  of  double  refraction,  and  from  amorphous 
bodies  by  the  presence  of  a  regular  boundary  or  by  the  occurrence  of 
rectilinear  cleavage  cracks  intersecting  one  another  at  uniform  angles. 
They  remain  dark  during  a  complete  rotation  in  parallel  light  between 
crossed  nicols,  give  no  interference  figure  in  convergent  light,  and 
exhibit  no  interference  colors  in  polarized  light.  Optical  anomalies 
are  frequent. 

Pyrite. 

Literature. 
FR.  BECKE,  Aetzversuche  am  Pyrit.    T.  M.  P.  M.  VIII.  239-337,  1887. 

Pyrite  is  occasionally  present  in  all  kinds  of  rocks  as  an  accidental 
accessory  constituent,  and  is  widely  distributed  in  small  quantities. 
It  forms  cubes,  pentagonal  dodecahedrons,  or  combinations  of  these 
forms  striated  by  the  oscillatory  combination  of  oo  O  co  (100)  with 

— ^ — n  (210) ;  less  frequently,  irregular  grains  and  aggregates  of  grains. 

It  is  opaque  and  yellow  by  incident  light,  with  strong  metallic 
lustre.  This  color  distinguishes  it  from  other  opaque  iron  ores. 

Sp.  gr.  =  4.9-5.2.  Chemical  composition  =  FeS2.  Soluble  in 
nitric  acid  with  the  separation  of  sulphur ;  not  noticeably  acted  on  by- 
hydrochloric  acid. 

Pyrite  is  often  intimately  associated  with  magnetite,  hematite,  and 
ilmenite.  It  is  often  peripherally  or  completely  pseudomorphosed 
into  limonite,  more  rarely  into  red  transparent  hematite.  Its  presence 
may  be  detected  during  the  grinding  of  the  section  by  its  dark  grayish- 
black  streak. 

Magnetite. 

Magnetite  forms  crystals  whose  predominant  form  is  O  (111),  and 
whose  dimensions  vary  greatly  in  one  and  the  same  rock ;  twins,  ac- 


MAGNETITE.  125 

cording  to  the  spinel  law,  often  very  much  shortened  in  the  direction  of 
the  twinning  axis;  and  simple  crys- 
tals grown  together  in  parallel  posi- 
tions. Cross-sections  of  such  forms  IMi  ^»~~  ^^ 
are  shown  in  Fig.  51.  Skeleton  crys-  ^^ 
tals  (PL  III.  Fig.  2)  are  frequent  in 
highly  ferruginous  eruptive  rocks. 
It  is  sometimes  quite  uniformly  dis- 
seminated through  the  rock  in  the  ^ 
form  of  grains  and  dust- like  particles, 

or  is  clustered  in  small  aggregates.  The  latter  occur  in  certain  eruptive 
rocks  (phonolites,  trachytes,  andesites,  tephrites),  where  they  often 
exhibit  a  more  or  less  close  approximation  to  the  crystal  forms  of 
hornblende,  biotite,  hypersthene,  more  rarely  of  augite ;  or  they  form 
in  combination  with  other  substances  complete  pseudomorphs  after 
the  minerals  just  named.  When  the  original  mineral  was  hornblende, 
biotite,  or  hypersthene,  the  aggregate  is  composed  of  magnetite  with 
augite  in  very  small  grains  and  crystals  (PL  XIY.  Fig.  1).  Such  pseu- 
domorphs appear  to  have  been  caused  by  the  resorption  of  the  older 
secretions  which  crystallized  at  a  particular  period  in  the  development 
of  the  magma,  but  which  at  a  later  period  could  no  longer  exist.  The 
cleavage  parallel  to  O  (111)  is  not  generally  perceptible  under  the 
microscope. 

Magnetite  is  opaque  in  rock  sections,  and  has  a  bluish-black  color 
by  incident  light  with  strong  metallic  lustre,  easily  recognized  except 
with  very  small  dimensions. 

Sp.  gr.  =  4.9-5.2.  Chemical  composition  —  FeO,  Fe2O3,  often 
containing  a  variable  amount  of  titanium,  rarely  with  a  small  amount 
of  chromium.  Dissolves  without  difficulty  in  hydrochloric  acid.  It 
remains  unattacked  in  a  rock  powder  treated  with  hydrofluoric  acid. 
It  is  easily  isolated  from  the  powder  by  means  of  a  weak  magnet. 
This  property  may  be  used  to  distinguish  it  from  hematite,  ilmenite, 
chromite,  and  graphite. 

Magnetite,  which  contains  no  titanium,  when  weathered  becomes 
coated  with  a  yellowish-brown,  non-metallic,  earthy  limonite,  which 
then  impregnates  the  surrounding  rock,  forming  a  halo  about  the  mag- 
netite. When  there  is  considerable  TiO2  present,  a  fibrous  or  granular, 
whitish  or  yellowish  substance  forms  around  the  magnetite,  which  is 
strongly  doubly  refracting,  and  is  the  same  mineral  which  often  accom- 
panies ilmenite,  titanite,  or  lencoxer.e.  Magnetite  is  frequently  crys- 
tallized with  pyrite  and  ilmenite,  more  rarely  with  chromite  and  rutile. 


126          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

No  other  mineral  is  so  generally  distributed  in  eruptive  rocks, 
crystalline  schists,  and  phyllites  as  magnetite.  In  the  eruptive  rocks  it 
belongs  to  the  oldest  crystalline  secretions  from  the  magma,  and  there- 
fore frequently  appears  as  inclusions  in  all  other  constituents,  espe- 
cially in  those  which  immediately  followed  its  formation,  as  olivine, 
biotite,  hornblende,  and  pyroxene.  Less  frequently  magnetite  is  of 
younger  origin  than  the  ferruginous  bisilicates,  when  it  very  probably 
arises  from  the  re-solution  of  older  basic  constituents. 

Chromite. 
Literature. 

E.  DATHE,  Olivinfels,  Serpentine  und  Eklogite  des  saclisischen  Granulitgebietes. 

K  J.  B.  1876.  247-249. 

J.  THOULET,  Note  sur  le  fer  chrome.     Bull.  Soc.  min.  Fr.  1879.  II.  34-37. 
M.  E.  WADSWORTH,  Lithological  Studies.     Cambridge,  Mass.,  1884,  pp.  176-186. 

Chromite  occurs  in  small  crystals  in  the  form  of  octahedrons,  sel- 
dom of  cubes  or  irregular  grains  and  aggregates ;  its  cross-sections  are 
quadratic,  rhombic,  triangular,  or  irregular.  Cleavage  not  noticeable  ; 
irregular  cracks  frequently  occur,  along  which  alteration  products  are 
often  located. 

In  sufficiently  thin  sections  it  is  transparent  with  brown  to  reddish- 
brown  color.  Its  highly  roughened  surface  arises  from  its  strong 
index  of  refraction,  n  =  2.0965.  Chromite  has  a  weak  metallic  lustre 
in  reflected  light  when  its  color  is  grayish  black  to  black,  and  it  is  not 
completely  transparent ;  but  there  is  no  metallic  lustre  on  transparent 
portions  which  are  of  a  gray  or  lilac-gray  color. 

Sp.  gr.  =  4.8  and  over;  hardness,  5.5.  These  are  important  in 
distinguishing  it  from  picotite.  Chemical  composition  =  FeO,  Cr2O3 
when  pure,  not  attracted  by  an  ordinary  magnet,  and  not  noticeably 
attacked  by  acids.  The  grains  of  chromite  in  the  rocks  are  often  sur- 
rounded by  a  green  halo  of  chrome  ochre. 

Chromite  is  common  in  crystalline  rocks  rich  in  magnesia  in  these 
it  belongs  to  the  oldest  secretions,  like  magnetite,  and  is  therefore 
usually  enclosed  in  the  next  oldest  constituents,  especially  in  olivine. 
Chromite  is  also  widely  disseminated  in  the  maguesian  rocks  of  the 
Archaean  formation,  particularly  in  serpentine.  Chromite  can  only  be 
distinguished  from  picotite  (chrome  spinel)  by  its  hardness,  specific 
gravity,  or  quantitative  analysis.  It  is  readily  distinguished  from  all 
other  substances  by  its  chemical  behavior,  especially  its  reaction  in 
blow-pipe  beads. 


SPINEL  GROUP,  127 

Spinel  Group. 

Literature. 

H.  FISCHER,  Kritische,  mikroskopisch-mineralogische  Studien.     Freiburg  i.  B.  1869. 
18;  I.  Fortsetzung  46,  60;  II.  Fortsetzung  66,  88. 

E.  KALKOWSKY,   Ueber    Hercynit    im  sachsischen  Granulit.     Z.    D.    G.  G.    1881. 

XXXIII.  533-539. 
W.  PRINZ,  Les  enclaves  du  saphir,  du  rubis  et  du  spinelle.     Ann.  Soc.   Beige  de 

Microscopic.  —  Bruxelles  1882. 
H.  SCHULZE  und  ALFR.  STELZNER,  Ueber  die  Umwandlung  der  Destillationsgefasse 

der  Zinkofen  in  Ziukspinell  und  Tridymit.     N.  J.  B.  1881.  I.  120-161. 
A.  STELZNER,  Zinkspinell-haltige    Fayalitschlacken  der  Freiberger  Huttenwerke. 

N.  J.  B.  1882.  I.  170-176. 
J.    THOULET,    Etude    microscopique  de  quelques  spinelles  naturels  et    artificiels. 

Bull.  Soc.  Min.  Fr.  1879.  II.  211-213. 
M.  E.  WADSWORTH,  Lithological  Studies.     Cambridge,  Mass.,  1884. 

F.  ZIRKEL,  Basaltgesteine.  —  Bonn  1870.  97. 

All  the  spinels  which  occur  as  rock  constituents  crystallize  in  sim- 
ple forms,  O  (111),  and  twins  of  such,  in  which  the  twinning  axis  is 
normal  to  the  octahedral  plane ;  more  rarely  in  combinations  of  O  with 
oo  O  (110),  and  3O3  (311).  Their  cross-sections  are  mostly  quadratic. 
Certain  spinels,  as  hercynite,  only  occur  in  irregular  grains;  all  of  them 
occur  in  this  form  very  frequently.  Cleavage  not  noticeable  micro- 
scopically. 

The  spinels  have  high  indices  of  refraction  (in  precious  spinels 
from  Ceylon  nna  —  1.7150),  and  exhibit  rough  surfaces  in  Canada  bal- 
sam. Their  color  in  transmitted  light  varies  with  their  chemical 
composition. 

H.  =  7.5-8;  sp.  gr.  =  3.6-4.5.  No  spinels  are  attacked  by  hydro- 
chloric and  hydrofluoric  acids ;  they  are  therefore,  in  general,  easily 
isolated  because  of  their  density  and  ability  to  withstand  chemical 
action. 

Spinel  proper ,  MgO,  A12O3,  is  colorless  to  light  red  by  transmitted 
light;  generally  in  sharply  defined  crystals.  It  occurs  sparingly  in 
the  Archaean,  especially  in  gneiss,  and  comes  into  river  sands  through 
the  mechanical  and  chemical  disintegration  of  these  rocks. 

Pleonaste  (MgO,  FeO)  (A12O3,  Fe2O3)  is  green  in  transmitted 
light,  but  by  incident  light  or  when  opaque  it  is  black  without  metallic 
lustre.  It  occurs  sometimes  with  magnetite  among  the  oldest  secre- 
tions of  many  eruptive  rocks.  It  is  extremely  common  in  gneisses, 
especially  in  those  intercalated  beds  bearing  cordierite  or  garnet,  in 
which  minerals  pleonaste  is  often  included.  In  these  occurrences  its 


128          PHYSIOGRAPHY   OF  THE  ROCK-MAKING  MINERALS. 

forms  are  usually  in-regular,  while  in  the  eruptive  rocks  it  is  mostly 
well  crystallized.  Its  occurrence  in  regions  of  contact  metamorphisrn 
is  specially  interesting. 

Hercynite,  FeO,  A12O3,  is  dark  green  by  transmitted  light,  and  can 
only  be  distinguished  from  pleonaste  through  isolation  and  chemical 
analysis.  It  occurs  in  the  granulites  of  Saxony,  and  in  norite  at 
Cruger's  Station,  on  the  Hudson  Kiver.* 

Gahnite  or  automolitk,  a  zinc  spinel,  is  also  green  when  trans- 
parent ;  it  occurs  sparingly  in  octahedrons  or  grains  in  crystalline  slates 
under  circumstances  analogous  to  those  accompanying  pleonaste,  from 
which  it  can  only  be  distinguished  chemically. 

Picotite,  a  chrome  spinel,  is  yellow  or  brown  by  transmitted  light, 
and  can  only  be  distinguished  from  chromite  chemically,  or  by  means 
of  its  density  and  hardness.  It  occurs  as  inclusions  in  the  olivine  of 
basaltic  rocks  in  minute  individuals  with  sharp  crystal  form ;  it  occurs 
very  rarely  as  an  independent  constituent  in  such  rocks,  as  was  ob- 
served in  the  basalt  of  Mount  Shasta,  California,  by  Wadsworth.f  In 
the  Iherzolites  and  olivine  rocks  it  is  more  often  found  in  irregular 
grains  than  in  crystals,  but  attains  greater  dimensions  ;  it  is  the  same 
in  the  serpentines. 

The  spinel  minerals  remain  perfectly  fresh  in  rocks,  all  the  rest  of 
whose  constituents  are  altered  and  decomposed. 


Fluorite. 

Fluorite  as  a  rock  constituent  does  not  appear  in  crystals,  but  in 
irregular  grains,  often  of  considerable  size.  The  cleavage  parallel 
O  (111)  forms  quite  sharp  systems  of  lines,  which  intersect  at  angles 
changing  with  the  position  of  the  section. 

Transparent,  clear,  bluish  or  violet  colored,  often  with  quite  un- 
equal distribution  of  the  pigment.  Its  very  low  index  of  refraction  is 
highly  characteristic ;  nna  — 1.4332-1.4340.  The  color  appears  to 
come  from  intermolecular  organic  matter,  hydrocarbons;  it  is  lost 
when  the  fluorite  is  heated  to  redness.  Traces  of  double  refraction  are 
rare  in  rock-making  fluorite. 

Sp.  gr.  =  3.18-3.20.  Its  specific  gravity  in  connection  with  its 
resistance  to  all  acids  but  sulphuric  facilitate  its  separation  from  a 

*  G.  H.  WILLIAMS:  The  Norites  of  the  "Cortlandt  Series,"  etc.     Ain.  Journ. 
Sci.,   Vol.  XXX1IL,  March,  1887. 

f  Harvard  University  Bulletin,  1882,  No.  22,  p.  259. 


GARNET  GROUP.  ^,) 

rock   powder.     Decomposition  products  are  wholly  wanting.     Fluicl- 
inclusions  are  frequent. 

Fluorite  occurs  only  as  an  accessory  constituent  in  recks;  in  giis'is  . 
granite,  quartz  porphyry,  syenite,  elaeolite  syenite  and  in  the  crystal- 
line schists. 

Garnet  Group. 

Literature. 

C.   KLEIN,    Optische   Studien  atn  Granat.     Nachr.  d.  kon.  Ges.  d.  Wiss.  zu  G5l- 

tingen.  1882.  No.  16.  457-564  and  K  J.  B.  1883.  I.  87-163. 
A.  VON  LASAULX,  Ueber  diellmrindungen  von  Granat.    Sitzungsbeiich.e  d  -  ::i»  d  :r- 

rhein.  Ges.  zu  Bonn.  1882.  3.  Juli. 
E.  MALLARD,  Explication  des  pheuomenes  optiques  anomaux  que  pre^  >      n  un 

grand  nombre  de  substances  cristallisees.     Paris.  1877.   (Ann.  dts  >J'ii;e,     7;   X 

60-203.  1876.) 
A.  RENARD,  Les  roches  grenatiferes  et  amphiboliques  de  la  region  de  .<»;.-.  gue 

Bulletin  du  Musee  Roy,  d'hist.  nat.  de  Belgique.     Bruxelles.  T.  I.  IbSi? 
A.  SCHRAUF,  Beitrage  zur  Kenntniss  des  Associationskreises  der  Jkagnefc»:a"i  itvaie. 

Z  X.  1882.  VI.  321-388. 
A.  WICHMANN,  Ueber  doppelbrechende  Granate.      Pogg.  Ann.   CLVI1.  282-290. 

1876.  —  Z.  D.  G.  G.  1875.  XXVII.  749. 

All   members   of   the  garnet  family  exhibit  very   simple   01 
forms  ;  those  found  in  rocks  have  mostly  the  forms  oo  O  ( ;.  i<)    aid  2O9 
(211),  alone  or  in  combination.     Their  cross-sections  ar.-  then  • 
ratic,  hexagonal,  or  eight-sided.     Irregular  grains  and  aggi    ;ates  (    ;.  r 
exclusively  in  many  r«fcks ;  in  others  they  are  accompai  '  d  b)      el 
defined  crystals.     Th*e  outlines  of  the  garnet  grains  arc  exceedingly 
irregular  in  some  Archaean  rocks.     Cleavage  is  not  noticeable  '.n  thin 
sections;  the  great  brittleness  of  the  mineral  gives  rise  to  .froii     i 
ular  fracturing. 

The  high  index  of  refraction  of  all  garnets  is  a  distirig 
acteristic.     nna  =  1.7468-1.8141.     The  dispersion  is  stron;  ie^itai 

pyrope  np  —  1.7TY6,  nv  =  1.8288.      The  rock-making   ga:  are    in 

general  completely  isotropic,  or  exhibit  very  faint  traces  c        uble  re- 
fraction.    Nevertheless   there  are  certain  occurrences,  e;    o,..ally  tlv 
lime-silicate  hornstories  and  the  garnet  rocks,  which  show     -i  .' 
double  refraction,  usually  connected  with  a  zonal  structur(     .;  , 
nets.     These  optical  anomalies  have  been  thoroughly  stud'i ;;,    a 
larly  by  E,  Mallard  and  C.  Klein. 

H.  =  Y-Y.5,  sp.  gr.  =  3.4-4.3,  varying  with  the  chemical  composi- 
tion.    Garnet  which  has   not  been  heated  to  redness  is  almost  wholly 
unacted  on  by  acids,  including  hydrofluoric,  and  it  is  decomposed  by 
9 


130          PHYSIOGRAPHY  OF,  THE  ROCK-MAKING  MINERALS. 

alkali  carbonates  only  after  long  fusion  with  very  fine  powder.  Fused 
garnet  is  decomposed  by  hydrochloric  acid  with  the  separation  of 
gelatinous  silica.  Its  chemical  behavior  and  high  specific  gravity 
greatly  facilitate  its  separation  from  a  rock  powder. 

From  the  great  tendency  of  all  garnets  to  form  isomorphic  lami- 
nae or  shells,  which  is  shown  by  a  difference  in  color  between  the 
"centre  and  the  margin,  or  between  alternating  shells  (PI.  Y.  Fig.  4), 
the  chemical  composition  seldom  corresponds  to  one  of  the  simple 
combinations  which  are  treated  in  mineralogy  as  the  varieties  of  gar- 
net. Moreover,  analytical  investigations  of  rock-making  garnets  are 
too  few  to  permit  of  giving  the  distribution  of  the  different  varieties 
in  the  rocks  with  certainty.  The  following  statements  therefore  may 
undergo  more  or  less  modification  : 

Grossular,  3CaO,  A12O3,  3SiOa,  transparent  and  colorless  or  nearly 
so  in  sufficiently  thin  sections.  Sp.  gr.  =  3.4-3.6.  Easily  fusible. 
Zonal  structure  and  optical  anomalies  frequent ;  sometimes  well  crys- 
tallized, oo  O  (110),  sometimes  in  grains  and  aggregates.  Occurs  espe- 
cially in  lime-silicate  hornstones,  and  as  inclusions  in  the  granular  lime- 
stone belts  of  the  Archaean,  also  in  garnet  rock  often  combined  with 
common  garnet  and  allochroite.  If  there  is  a  zonal  difference  in  its 
color,  the  centre  is  darker  than  the  margin.  It  frequently  enclose& 
fluid  inclusions,  calcite,  quartz,  wollastonite,  epidote,  vesuvianite, 
and  graphite.  Decomposition  products  unknown. 

Almadine,  3FeO,  A12O3,  3SiO2,  is  red  by  transmitted  light.  Sp. 
gr.  =  4.1-4.3.  Easily  fused  to  a  dark  magnetic  bead.  It  occurs  as 
grains  in  many  granitic  rocks,  seldom  in  crystals,  2O2  (211) ;  it  also 
OCCIHTS  in  the  Hungarian  andesites  in  grains  and  crystals.  It  is  most 
abundant  in  the  gneisses,  granulites,  and  in  those  Archaean  rocks  free 
from  feldspar ;  generally  as  grains,  more  rarely  in  crystals,  which  have 
202  (211)  predominant  in  rocks  rich  in  feldspar,  and  oo  O  (110)  pre- 
dominant in  those  poor  in  feldspar.  It  usually  encloses  the  minerals 
associated  with  it.  Its  substance  is  generally  extremely  fresh.  It  is 
found  altered  to  chlorite  at  Spurr  Mountain  Iron  Mine,  from  which 
locality  it  has  been  described  by  Pumpelly,*  and  more  recently  it  has 
been  thoroughly  investigated  by  Penfield  f  and  Sperry,  who  also  de- 
scribed a  similar  alteration  of  iron-alumina  garnet  from  Salida,  Chaff ee 
County,  Colorado. 

Common  garnet,  an  isomorphic  mixture  of   the  grossular,  alma- 

*  Amer.  Journ.  Sci.  (3)  X.  17.  July,  1875. 
f  Amer.  Journ.  Sci.  (3)  XXXII.  Oct.  1886. 


GARNET  GROUP.  131 

dine,  and  melanite  molecule,  occurs  in  certain  garnet  rocks,  in  a  meta- 
morphosed eruptive  rock  of  the  diabase  and  gabbro  series,  in  Archaean 
rock,  especially  in  kirizigite,  eulysite,  amphobolite  eclogite,  pyroxene 
rocks,  and  their  derivatives,  as  well  as  in  the  phyllite  formations.  It  is 
reddish  brown  to  yellowish  red  by  transmitted  light,  often  nearly  color- 
less. Zonal  structure  is  very  common,  often  accompanied  by  optical 
anomalies.  The  crystal  form  is  generally  wanting.  Inclusions  of  the 
associated  minerals  and  fluid  inclusions  centrally  accumulated  are  fre- 
quent ;  so  is  also  a  micropegmatitic  intergrowth  with  the  associated 
minerals,  and  their  radial  arrangement  about  the  garnet  as  a  centre. 
Decomposition  products  are  not  uncommon.  One  form  is  the  pseudo- 
morph  of  chlorite  after  garnet,  described  by  Hawes  *  from  the  phyl- 
lites  of  the  Connecticut  Yalley  in  New  Hampshire.  Its  alteration 
into  hornblende  has  been  quite  often  observed,  and  in  one  instance  into 
scapolite. 

Lime-iron  garnet.  3CaO,  Fe2O3,  3SiO2,  in  velvet-black  crystals 
with  the  form  oo  O  (110)  2O2  (211),  called  melanite,  frequently  occurs 
as  an  accessory  constituent  in  those  basic  eruptive  rocks  rich  in  alkali 
(phonolite,  leucitophyre,  nephelinite,  tephrite).  It  is  brown  in  trans- 
mitted light  with  various  depths  of  color,  and  forms  one  of  the  oldest 
secretions.  Optical  anomalies  are  rare.  Decomposition  phenomena 
are  wanting.  Melanite  accompanied  by  wollastonite  and  fassaite  has 
been  described  by  Fouque  f  as  a  volcanic  contact  phenomenon. 

Green  lime-iron  garnet  occurs  in  many  serpentines ;  it  sometimes 
•shows  a  zonal  structure  of  green  and  red  layers.  It  is  brown  in  many 
iron-ore  beds  in  the  crystalline  schists.  The  lime-iron  garnets  have 
sp.  gr.  =  3.4-4.1,  and  fuse  to  a  strongly  magnetic  bead. 

Spessartine,  essentially  3MnO,  A12O3,  3SiO2,  occurs  occasionally  in 
granitic  rocks  in  the  form  of  grains  off  considerable  size.  Its  color  is 
sometimes  blood-red,  sometimes  yellowish  red  to  colorless.  Its  decom- 
position processes  are  not  known.  It  occurs  with  topaz  in  the  litho- 
physae  of  rhyolite  from  Nathrop,  Colo.,  according  to  Cross.} 

Pyrope,  principally  3MgO,  A12O3,  3SiO2,  with  some  chromium 
and  a  variable  admixture  of  the  almadine  molecule,  never  forms 
crystals,  but  angular  to  rounded  grains  of  red  or  blood-red  color  by 
transmitted  light,  and  mostly  of  very  pure  substance.  It  fuses  with 
difficulty  to  a  non-magnetic  bead.  Sp.  gr.  =  3.7-3.8.  Pyrope  appears 

*  Mineralogy  and  Lithology  of  New  Hampshire.     Concord,  1878.  75. 

\  Compt.  rend.  1875.  15  Mars. 

JAmer.  Jour.  Sci.,  Vol.  XXXI.,  June,  1886,  p.  432. 


132          PHYSIOGRAPHY  OF  THE  ROCK-MAKING   MINERALS. 

to  be  confined  to  rocks  rich  in  magnesia:  the  periclotites  arid  their 
derivatives,  the  serpentines.  In  these  rocks  the  pyrope  is  often  sur- 
rounded by  a  radial  grouping  of  the  other  constituents,  especially  the 
pyroxenes  and  their  alteration  products.  Yery  frequently  the  pyrope 
in  serpentine  is  surrounded  by  a  radially  fibrous,  light  grayish-brown 
shell,  which  is  an  alteration  product  of  the  pyrope,  probably  with  the 
co-operation  of  the  olivine  substance.  This  shell  has  been  called 
Icelyphite  (PL  XIY.  Fig.  4) ;  its  composition  is  not  constant,  and  its 
mode  of  formation  is  still  doubtful.  Diller  *  has  described  kelyphite 
shells  of  biotite  and  magnetite  around  pyrope  in  the  peridotite  from 
Elliott  Co.,  Ky. 

Leucite. 

Literature. 

H.  BAUMHAUER,  Studien  iiber  den  Leucit.    Z.  X.  1877.  I.  257-273. 

A.  DBS  CLOIZEAUX,  Nouvelles  recherches  sur  les  proprietes  optiques  des  cristaux 

naturels  ou  artificiels  et  sur  les  variations  que  ces  proprietes  eprouvent  sous 

1'influence  de  la  chaleur.     Paris.  1887.  513-515. 

J.  HIRSCHWALD,  Zur  Kritlk  des  Leucitsystems.     T.  M.  M.  1875.  IV.  227  ff. 
—  Ueber  unsere  derzeitige  Kenntniss  des  Leucitsystems.     T.  M.  P.  'M.  1878.  II, 

85-100. 
C.  KLEIN,  Optische  Studien  am  Leucit.     Gottinger  gelehrte  Nachrichteu  1884.  No. 

11.  421-472.  —  N.  J.  B.  1885.  Beil.-Bd.  III.  522-584. 

E.  MALLARD,  Explication  des  phenomenes  optiques  anomaux  que  presentent  un 

grand  nombre  de  substances  cristallisees.     Paris.  1877.  24-39.  (Ann.  des  Mines 

(7).  X.  1876.) 
G.  VOM  RATH,  Ueber  das  Krystallsystem  des  Leucits.     M.  B.  A.  1872.  1  Aug.  — 

Pogg.  Ann.  Erganzungsband  VI.  1872.  198  ft.  and  Sitzungsber.  der  niederrhein. 

Ges.  Bonn.  4.  Juni  1883. 
A.  WEISBACH,  Zur  Kenntniss  des  Leucits.     1ST.  J.  B.  1880.  I.  143-150. 

F.  ZIRKEL,  Ueber  die  mikroskopische  Structur  der  Leucite.     Z.  D.  G.  G.  1868.  XX. 

97  sqq.  —  Basaltgesteine  1870.  pg.  44  ff . 

The  crystal  system  of  leucite  has  been  the  subject  of  much  discus- 
sion and  great  uncertainty  for  many  years.  Its  habit,  without  excep- 
tion, is  that  of  an  isometric  crystal,  exhibiting  the  icositetrahedron, 
2O2  (211),  alone,  or  with  oo  O  (110)  and  oo  O  oo  (100)  less  strongly  de- 
veloped. And  notwithstanding  its  anomalous  optical  behavior,  it  was 
considered  an  isometric  body  by  Brewster,f  Biot,J  and  Des  Cloizeaux 
(1.  c.)  until  the  year  1872. 

The  polarization  phenomena  were  explained  as  lamellar  polariza- 

*  Bull.  38,  U.  S.  Geol.  Surv.  1887,  p.  15. 

f  Edinburgh  Phil.  Journ.  1821.  V.  218. 

\  Memoire  sur  la  polarisation  lamellare.  1841.  669. 


LEUCITE.  133 

tion,  or  as  the  effect  of  intercalated  lamellae.  G.  vom  Kath,  in  1872, 
placed  it  among  the  tetragonal  minerals  as  the  result  of  his  study  of 
the  twinning  striae  on  the  crystal  faces  and  of  the  values  of  the  angle 
of  the  apparent  2O2  (211)  edges.  Hirschwald,  on  the  ground  of  its 
crystal  habit  and  of  the  twinning,  which  is  parallel  to  the  6  .faces  of 
oo  O,  restored  it  to  the  isometric  system ;  while  Weisbach  considered 
it  orthorhombic.  Investigating  the  mineral  by  physical  methods, 
Baumhauer  concluded  that  the  result  of  etching  its  crystal  faces  indi- 
cated its  tetragonal  nature,  or  at  least  did  not  militate  against  such  an 
assumption.  Mallard's  investigation  of  the  optical  behavior  of  sections 
parallel  to  the  cubical  faces  (supposing  leucite  isometric)  led  him  to 
refer  it  to  the  monoclinic  system.  C.  Klein  was  convinced  by  his 
study  of  leucite  that  under  the  physical  conditions  acompanying  its 
formation  it  had  crystallized  isometrically,  but  that  its  molecular 
structure  at  ordinary  temperatures  may  be  considered  orthorhombic. 
Summing  up  the  results  of  all  leucite  studies  down  to  the  present 
time,  it  may  be  confidently  stated  that  all  leucite  crystallized  in  the 
isometric  system,  but  that  the  isometric  molecular  arrangement,  at 
least  of  the  larger  crystals,  cannot  obtain  for  the  temperatures  and 
pressures  at  the  earth's  surface ;  that  it  therefore  experiences  a  molecu- 
lar displacement,  in  consequence  of  which  there  arises  a  more  or  less 
complicated,  apparent  twinning.  This  molecular  displacement  has  not 
only  an  optical  effect,  but  leads  to  a  more  or  less  profound  deformation 
of  the  crystal  form.  It  is  not  yet  possible  to  decide  from  the  goni- 
ometric  and  optical  behavior  of  leucite  to  which  crystal  system  this 
molecular  displacement  tends  to  change  it.  - 

Leucite  furnishes  an  excellent  example  of  the  group  of  minerals  in 
which  optical  anomalies  are  occasioned  by  dimorphism. 

The  cross-sections  of  leucite  crystals  are  six-sided,  eight-sided,  or 
rounded,  according  to  the  greater  or  less  development  of  2O2 ;  there 
also  occur  quadratic,  triangular,  or  rhombic  sections  from  the  surface 
of  the  crystals  parallel  to  (100),  (111),  (110).  The  larger  individuals 
frequently  exhibit  irregularities  of  outline  due  to  the  corrosion  of  the 
crystals,  and  sometimes  appear  to  be  composed  of  a  number  of  smaller 
crystals.  Leucite  crystals  vary  greatly  in  size ;  massive  leucite  appears 
to  be  extremely  rare. 

Cleavage  is  not  noticeable  in  thin  section  ;  but  a  cracking  of  the 
crystals  along  irregular  faces  is  very  often  present,  as  the  result  of 
molecular  shifting. 

The  very  small  crystals  of  leucite  appear  wholly  isotropic  when 
investigated  optically ;  in  the  larger  individuals  a  very  complicated 


134 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


twin  lamination  is  observed  between  crossed  nieols,  especially  upon  the 
insertion  of  a  gypsum  plate ;  these  laminse  are  doubly  refracting,  and 
intersect  at  angles  which  change  with  the  position  of  the  section.  The 
index  of  refraction  of  leucite  is  low,  and  differs  very  little  from  that 
of  a  rock  glass.  The  double  refraction  is  weak,  and  positive  according 
to  Des  Cloizeaux  (<»  =  1.508,  e  =  1.509)  and  Klein.  Tschermak* 
found  its  character  negative  in  one  instance. 

In  very  thin  sections  it  is  necessary  to  use  a  sensitive  tint  in  order 
to  perceive  the  anisotropism  and  to  study  the  twinning.  The  inter- 
ference colors  in  thin  sections  do  not  exceed  grayish  blue  of  the  1st 
order.  This  double  refraction,  and  with  it  the  twin  lamination,  disap- 
pear when  the  crystals  are  exposed  to  a  temperature  of  about  500° 
C.,  when  they  appear  isotropic  as  first  observed  by  Klein,f  and  sub- 
sequently by  Penfield.^:  Rosenbusch  §  showed  that  at  this  tempera- 
ture the  twinning  striae  on  the  crystal  faces  disappear,  so  that  they 
reflect  light  with  perfect  uniformity,  from  which  it  is  very  probable 
that  they  also  return  to  isometric  symmetry  in  a  goniometrical  sense. 

The  twinned-like  structure  of  leucite  has  been  thoroughly  studied 
by  Klein,  whose  results  are  given  with  illustrations  by  Rosenbusch, 
but  are  here  omitted. 

The  larger  crystals  of  leucite  usually  enclose  the  older  secretions 
associated  with  them ;  these  minerals  are  magnetite,  picotite,  apatite, 
olivine,  augite,  haiiyne,  nepheline,  and  melanite.  More  frequently  the 
inclusions  are  prismatic  microlites,  which  are  in  part  green  and  most 
likely  augite,  and  in  part  colorless  and  indeterminable.  These  are 

either  crowded  at  the  centre  of  the 
leucite,  or  are  arranged  in  concen- 
tric zones,  Fig.  52 ;  they  then  lie  with 
their  longer  axis  parallel  to  the  bound- 
ary of  the  enclosing  mineral.  These 
are  often  accompanied  by  glass  inclu- 
sions and  gaseous  interpositions,  and 
more  rarely  by  those  of  a  fluid  (Capo 
di  Bove,  Monte  Vulture,  Olbriick). 
The  glass  inclusions  take  the  form  of 
the  enclosing  mineral.  Sometimes  all 
the  inclusions  in  one  crystal  are  of  the 
same  kind;  sometimes  the  different  kinds 
lie  together  indiscriminately  ;  at  others  they  are  so  arranged  that  zones 


Figv  53 


*  T.  M.  M.  1876.  66. 
f  K  J.  B.  1884.  II.  50. 


\  K  J.  B.  1884.  II.  224. 
§  N.  J.  B.  1885.  II.  59. 


80DALITE  OHO  UP.  135 

of  different  kinds  alternate  witli  one  another.  "Very  rarely  there  is  a 
radial  arrangement  of  the  interpositions,  or  a  combination  of  radial  and 
zonal  arrangement  in  the  same  crystal  (PI.  XIV.  Figs.  5  and  6). 
Very  small  lencite  crystals  are  usually  free  from  interpositions.  Leucite 
crystals  are  often  encircled  by  a  veil  or  shell  of  augite  microlites  (PI. 
XV.  Fig.  1),  which  is  at  times  a  means  of  recognizing  the  leucite  when 
the  characteristic  twinning  is  not  noticeable. 

Sp.  gr.  =  2.45-2.5.  Leucite,  K2O,  A12O3,  4SiO2,  mostly  with  no  very 
considerable  percentage  of  Na3O,  is  very  slightly  attacked  even  by  hot 
hydrochloric  acid  when  in  thin  section,  but  as  powder  it  is  stronglv 
attacked  with  the  separation  of  pulverulent  silica.  The  mineral  is 
therefore  better  isolated  from  the  rock  by  specific-gravity  methods 
than  by  chemical  ones. 

Leucite  alters  quite  frequently  into  analcite  without  the  form  of 
the  crystal  being  changed  in  any  way.  Nevertheless  there  is  formed 
as  a  side-product  radially  and  confusedly  fibrous,  double  refracting 
aggregates  of  an  indeterminable  nature,  which  are  often  in  considerable 
quantity.  The  analcite,  in  turn,  is  altered  to  a  mixture  which  is  prin- 
cipally feldspar  and  light-colored  mica. 

Leucite  is  a  mineral  wholly  confined  to  Tertiary  and  recent  erup- 
tive rocks  and  their  tuffs;  it  accompanies  sanidine  and  nepheline  in 
rocks  of  the  phonolite  series,  plagioclase  and  nepheline  in  the  tephritic 
rocks,  and  nepheline  alone  in  the  leucite  basalts  and  leucitites. 

Zirkel*  has  described  its  occurrence  at  Leucite  Hills,  Wyoming 
Ter.  ;  and  von  Chrustschoff  f  has  found  it  in  leucite  porphyry  from 
Cerro  de  las  Virgines,  in  Lower  California. 


Sodalite  Group. 

Literature. 

L.  L.  HUBBARD,  BeitrSge  zur  Kenntnis  der  Nosean-  f uhrenden  Auswiirflinge  des 

Laacher  Sees.     T.  M.  P.  M.  VIII.  1887.  356-399. 
B.  MIEBISCH,  Die  AuswurfsblOcke  des  Monte  Somma.      T.  M.  P.  M.  VIII.  1887. 

113-189. 
G.  VOM  RATH,  Mineralogisch-geognostische  Fragmente  aus  Italien.      Z.  D.  G.  G. 

1866.  XVIII.  620-624. 
—  Skizzen  aus  dem  vulkanischen  Gebiet  des  Niederrheins.     Z.  D.  G.  G.  1860.  XII. 

29  ;  1862.  XIV.  663  ;  1864.  XVI.  73. 
A.  SALTIER,  Untersuchungen  liber  phonolithische  Gesteine  der  canarischen  Inseln. 

Halle.  1876.     Zeitschr.  f.  d.  ges.  Naturw.  XL VII. 

*  Microscopic  Petrography.     Washington,  1876. 
t  T.  M.  P.  M.  Vol.  VI.  1885,  pp.  160-171. 


136         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

H.  VOGELSANG,  Ueber  die  natlirlichen  Ultramarinverbindungen.    Kon.  Akad.  van 

Wentesch.  Amsterdam  (2)  VII.  1873. 
F.  ZIKKEL,  Untersucliungen  Uber  die  mikroskopisclie  Zusammensetzung  und  Struk- 

tur  der  Basaltgesteine.    Bonn.  1370.  79  ff. 

The  sodulite  group  includes  a  number  of  minerals  crystallizing  in 
the  isometric  system,  sodalite,  haiiyne,  with  the  varieties  nosean,  ittner- 
ite  and  skolopsite,  and  lapis-lazuli.  They  are  characterized  chemically 
by  the  fact  that  they  present  isomorphous  combinations  of  a  silicate 
molecule  with  the  salt  of  another  acid,  or  with  a  haloid  compound. 
The  first-named  substances  are  widely  spread  rock-making  minerals  of 
extremely  characteristic  geological  position. 

Sodalite. 

Sodalite  forms  simple  crystals,  more  rarely  twins  according  to  the 
spinel  law.  When  they  occur  in  porphyritic  rocks  they  are  rhombic 
dodecahedrons  and  octahedrons,  either  alone  or  in  combination  with 
one  another.  In  granular  rocks  their  forms  become  more  indistinct 
or  are  lost  altogether  ;  between  the  sodalite  and  the  younger  rock  con- 
stituents the  former  shows  its  crystal  outline ;  between  it  and  the  older 
constituents  the  outline  is  that  of  the  older  secretions.  Cross-sections 
of  the  crystals  are  mostly  quadratic  or  hexagonal,  but  are  often  dis- 
torted by  the  crystals  being  strongly  developed  in  the  direction  of  a 
trigonal  secondary  axis. 

In  the  freely  crystallized  sodalite  crystals  of  Monte  Sornrna,  the 
cleavage  parallel  to  ooO  is  very  clearly  perceptible,  even  in  thin  sec- 
tion ;  the  sodalites  crystallized  within  rocks  exhibit  much  less  typical 
cleavage. 

In  transmitted  light  sodalite  is  colorless  ;  also  bluish,  greenish,  light 
pink,  red,  and  yellowish.  Its  index  of  refraction  is  low  ;  nna  =  1.4827- 
1.4858.  Optical  anomalies  have  been  observed  occasionally  in  the 
vicinity  of  inclusions. 

Sp.  gr.  =  2.28-2.34.  Chemical  composition  =  2(Na3O,  AlaO3,  2SiOa) 
-f-  NaCl.  Easily  and  completely  soluble  in  hydrochloric  arid  nitric 
acids  ;  upon  standing  gelatinous  silica  separates  out ;  it  is  even  acted 
on  in  thin  section  by  acetic  acid.  In  order  to  observe  the  gelatiniza- 
tion  well  in  thin  section  it  should  be  moistened  with  only  a  very  thin 
coat  of  acid.  If  the  mineral  has  been  treated  with  hydrochloric  acid 
there  will  be  an  abundance  of  common  salt  crystals  when  the  jelly  dries. 

Sodalite  is  quite  a  constant  constituent  of  eleeolite-syenites  ;  in  these 
rocks  it  occurs  in  crystals,  grains,  and  massive  forms,  or  in  veins  arid 


80DAL1TE.  137 

streaks  within  other  minerals,  especially  feldspar.  The  formation  of 
sodalite  in  these  rocks  followed  the  secretion  of  the  iron-bearing  con- 
stituents and  preceded  that  of  the  feldspar.  Its  age  relative  to  that  of 
eltfioiite  appears  to  vary.  The  primary  nature  of  sodalite  in  these  rocks 
is  in  general  beyond  question.  Only  the  vein-like  massive  occurrences 
are  possibly  of  secondary  origin.  In  the  rocks  mentioned  sodalite 
often  encloses  the  ores,  and  needles  of  pyroxene  and  hornblende  asso- 
ciated with  it,  besides  fluid-inclusions  ;  these  when  abundant  are  usually 
accumulated  centrally. 

Upon  the  alteration  of  sodalite,  which  is  sometimes  very  far 
advanced  in  these  rocks,  there  arise  tufted  aggregates  of  zeolites ; 
spreustein,  according  to  Brogger,  is  a  pseudomorph  of  natrolite  after 
sodalite.  In  other  cases  there  arise  aggregates  of  rnuscovite  and  kaolin. 
Carbonates  which  are  not  infrequently  found  along  cracks  in  sodalite 
are  evidently  infiltration  products. 

Sodalite  occurs  only  in  sharp  and  distinct  crystals  in  the  younger 
rocks  of  the  trachyte  and  phonolite  families.  It  is  wide-spread  in  the 
trachytes  of  the  island  of  Ischia  .  it  is  found  sparingly  in  the  place  of 
haiiyne  in  many  phonolites  of  Northern  Africa  and  of  the  Cantal  (Pas 
de  Compains),  as  also  in  the  granular  tephrite  of  the  Crazy  Mountains, 
Montana  Territory.* 

Its  formation  follows  that  of  augite  without  exception,  and  precedes 
that  of  nepheline  or  feldspar.  Inclusions  are  rare ;  they  consist  of 
the  older  minerals  which  accompany  it,  especially  augite,  together  with 
glass  and  rarely  fluid  inclusions. 

Its  isotropism  in  connection  with  the  low  index  of  refraction  and 
its  ready  solubility  in  acids,  as  well  as  its  low  specific  gravity,  distin- 
guish sodalite  readily  from  all  other  minerals,  with  the  exception  of 
haiiyne  and  nosean.  These  are  characterized  by  their  chemical  be- 
havior, giving  reaction  for  sulphuric  acid. 

Hauyne  and  Nosean. — All  those  members  of  the  sodalite  group  are 
classed  under  the  name  hauyne^f\\\o\\  are  considered  isomorphous  mix- 
tures of  2(Na2O,  A12O3,  2SiO2)  +  Na2O,  !3O3and2(CaO,  A12O3,  2SiO2) 
+  CaO,  SO3.  The  first  of  these  compounds  is  found  in  a  nearly  pure 
condition  in  the  volcano  Siderao,  in  the  Cape  Verde  Islands.f  The 
second  compound  is  not  known  to  occur  alone.  The  analyses  as  well  as 
the  micro-chemical  reactions  very  frequently  give  a  slight  percentage 


*  J.  E.   Wolff.     Notes  on   the  Geology  of   the  Crazy  Mountains.     Northern 
Transcontinental  Survey.  1885.     c.f.  N.  J.  B.  1885.  I.  69. 

f  C.  Doelter,  Die  Hauyne  der  Capverden.     T.  M.  P.  M.  1882.  IV.  461. 


138  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

of  chlorine,  which  indicates  the  presence  of  an  isomorphous  admixture 
of  the  sodalite  compound. 

The  members  of  this  group  rich  in  soda  are  called  nosean,  in  dis- 
tinction to  those  rich  in  lime,  called  hauyne.  All  the  members  of  this 
series  gelatinize  easily  with  acids :  if  the  gelatinous  silica  be  allowed 
to  dry  under  the  microscope,  or  better,  if  the  solution  obtained 
through  the  action  of  acids  be  removed  and  allowed  to  evaporate^ 
there  arise  in  the  case  of  baiiyne  abundant  characteristic  gypsum  crys- 
tals ;  if  the  mineral  was  nosean,  these  would  appear  but  sparingly,  or 
not  at  all.  The  hydrochloric  acid  should  not  be^  too  concentrated, 
nor  the  temperature  too  high,  as  in  this  case  orthorhombic  cube-like 
crystals  of  anhydrite  will  be  produced  in  place  of  gypsum.  The  color 
is  changed  in  many  occurrences  by  heating  to  redness,  or  the  crystals- 
may  become  colored  if  they  were  colorless  before.  They  are  colored 
blue  when  heated  to  redness  in  vapor  of  sulphur. 

The  sp.  gr.  varies  from  2.27-2.50,  according  to  the  chemical  com- 
position and  to  the  greater  or  less  abundance  of  interpositions.  Nosean 
is  lighter  than  baiiyne ;  but  this  may  be  obscured  by  the  presence  of 
included  hematite  or  ilmenite  plates.  The  cleavage  parallel  GO  0  (110) 
is  rarely  observed  microscopically. 

The  haiiynes  crystallized  in  rocks,  when  fresh,  are  generally  per- 
fectly isotropic ;  but  there  are  occurrences  which  exhibit  optical  anom- 
alies (Vesuvius,  Lake  Laach).  These  are  of  two  kinds :  in  the  first 
case  there  is  a  local  double  refraction,  which  only  occurs  around  inclu- 
sions, especially  about  gas  inclusions ;  it  is  characterized  by  a  dark  cross 
between  crossed  nicols,  the  arms  lying  parallel  to  the  principal  sections 
of  the  nicols,  and  remaining  so  during  a  rotation  of  the  section. 
The  gas  inclusion  is  at  the  centre  of  the  cross,  and  gives  rise  to  the 
phenomenon  by  the  pressure  which  it  exerts  on  the  surrounding 
mineral.  The  elasticity  is  greater  in  radial  directions  about  the  in- 
clusions than  in  tangential  directions.  Similar  anomalies  are  often 
observed  along  cracks  in  the  baiiyne. 

The  second  kind  of  optical  anomaly  is  a  double  refraction  through- 
out the  whole  extent  of  the  mineral  (Vesuvius ;  Lake  Laach,  Nieder- 
mendig,  Rhine  Province) :  this  is  always  weak,  often  only  noticeable 
by  the  use  of  a  gypsum  or  quartz  plate. 

The  index  of  refraction  of  baiiyne  is  low,  but  somewhat  higher 
than  that  of  sodalite.  nna  =  1.4961.  The  color  is  extremely  mani- 
fold :  baiiyne  is  colorless,  blue,  gray,  brownish,  red,  yellow,  and  green  ; 
and  the  color  is  often  irregularly  distributed  in  spots,  streaks,  and 
stripes,  or  in  concentric  zones  in  thin  section.  Occasionally  the  color 


SO  DA  LITE.  139 

is  most  intense  along  the  cracks,  and  is  most  easily  developed  here  in 
colorless  individuals.  This  led  Vogelsang  to  conclude  that  the  color  is 
of  secondary  nature.  Yery  strong  heating  destroys  the  original  or 
artificial  color  of  haiiyne. 

The  haiiynes,  with  the  exception  of  ittnerite  and  skolopsite,  never 
occur  in  irregular  masses,  but  always  in  crystals  or  crystal  fragments  or 
in  grains  (rounded  crystals).  The  forms  and  cross-sections  are  the  same 
as  those  of  sodalite.  The  microstructure  of  haiiyne  is  very  variable. 
Most  all  occurrences  abound  in  inclusions.  These  are  usually  the  iron- 
ores,  oxides,  gas  and  glass  inclusions;  fluid  inclusions  in  great  numbers 
and  of  very  different  forms  are  confined  to  particular  localities  (Nieder- 
mendig).  All  the  foreign  bodies,  for  the  most  part,  are  present  at  the 
same  time,  either  scattered  irregularly  through  the  mineral  substance, 
or  regularly  arranged,  especially  when  their  quantity  is  considerable. 
In  the  latter  case  they  are  sometimes 
aggregated  in  the  centre  or  periph- 
erally; sometimes  they  are  aggre- 
gated in  concentric  shells.  More- 
over, the  interpositions  are  often 
arranged  in  lines  parallel  to  the 
octahedral  axes.  Cross-sections  par- 
allel to  oo  6>  oo  (100)  and  oo  O  (110) 
exhibit  two  systems  of  lines  inter- 

* 

secting  at  right  angles,  while  in  sec- 
tions parallel  0  (111)  there  are  three  systems  intersecting  at  60°  (Fig. 
53).  The  glass  and  gas  inclusions,  when  large,  frequently  have  the 
crystal  form  of  the  enclosing  mineral. 

The  noseans  in  the  leucite  porphyries  of  the  volcanic  territory  of 
the  lower  Ehine  often  have  a  broad  opaque  border,  having  a  bluish- 
black  or  brownish-black  color.  This  probably  arises  from  the  conver- 
sion of  the  iron-bearing  compounds  in  a  zone  rich  in  interpositions 
into  limonite,  and  the  possibly  contemporaneous-  kaolinization  of  the 
haiiyne  substance. 

The  haiiyne  minerals  are  very  often  found  in  nature  in  a  more  or  les& 
advanced  stage  of  alteration.  There  are  probably  two  processes  which 
can  be  distinguished— the  zeolitization  and  the  weathering  proper.  The 
zeolitization,  which  takes  place  through  the  addition  of  water  and  the 
loss  of  the  sulphates  from  the  molecule,  shows  itself  very  quickly  by 
the  formation  of  a  fibrous  structure.  These  grayish  to  yellowish  fibres 
penetrate  the  clear  mineral  substance  in  the  form  of  bundles,  starting 
from  cracks,  from  the  surface,  and  from  the  larger  interpositions.  In 


140  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

consequence  of  the  anisotropic  nature  of  the  zeolite  fibres  they  are 
noticeable  between  crossed  nicols.'  When  the  alteration  is  complete 
the  whole  section  consists  of  bundles  of  fibres  radiating  from  different 
points.  The  zeolite  resulting  from  haiiyne  is  usually  natrolite.  The 
cloudy  coloring  of  this  natrolite  aggregate  is  not  due  to  a  pigment, 
but  is  mostly  the  result  of  its  extraordinarily  fine  fibrous  aggregation. 
That  other  zeolites  than  natrolite,  especially  stilbite  and  chabazite,  may 
result  from  the  alteration  of  the  haiiynes  rich  in  lime  is  very  probable, 
both  from  the  microscopical  habit  and  also  from  the  microscopical  oc- 
currence of  these  minerals  in  rocks  rich  in  haiiyne ;  but  the  direct  proof 
of  it  has  not  yet  been  produced.  Very  often  the  lime  component  of 
the  original  mineral  is  secreted  in  the  form  of  calcite  when  it  is  altered 
to  natrolite.  Zeolitized  haiiyne  becomes  clouded  when  heated  to  red- 
ness through  the  loss  of  water. 

The  distribution  of  haiiyne  in  rocks  is  very  great.  However,  it  is 
confined  to  rocks  of  the  youngest  geological  periods  which  are  poor  in 
silica  and  rich  in  alkali:  in  this  it  is  distinguished  from  sodalite,  whose 
formation  goes  back  to  the  Palaeozoic  period.  The  true  phonolytes 
and  leucite  porphyries  contain  it  almost  without  exception;  it  is  found 
with  less  regularity  in  the  tephrytes,  the  nepheline  and  leucite  rocks, 
and  in  their  varieties  free  from  feldspar.  Nosean  occurs  in  the  phono- 
lite  at  Black  Hills,  Dakota.  In  many  nepheline  rocks  the  amount  of 
haiiyne  can  become  so  excessive  that  it  forms  the  next  principal  con- 
stituent to  pyroxene,  and  supplants  the  nepheline :  such  rocks  haye  been 
called  haiiynophyres.  In  all  these  varieties  of  rocks  the  formation  of 
haiiyne  in  the  molten  magma  followed  that  of  the  older  pyroxenes,  and 
preceded  that  of  nepheline ;  it  is  therefore  the  oldest  of  the  feldspathic 
components.  In  all  the  rocks  above-named  haiiyne  is  associated  with 
nepheline,  or  with  nepheline  and  leucite ;  the  only  rocks  in  which  it 
occurs  without  these  minerals  are  certain  andesites  of  the  Canary  Islands. 

Ittnerite  and  skolopsite,  which  correspond  microscopically  and 
chemically  to  haiiyne,  have  only  been  found  in  one  locality  in  the 
Kaiserstuhl. 

Anal  cite. 

Literature. 

A.  BEN  SAUDE,  Ueber  den  Analcim.    N.  J.  B.  1882.  I.  41-79. 
C.  KLEIN,  Analcim  vom  Table  Mountain  bei  Golden,  Col.     K  JVB.  1884.  I.  250. 
E.  MALLARD,  Explication  des  phenomenes  optiques  anomaux  que  presentent  un 
grand  nombre  de  substances  cristallisees.     Paris.  1877.  57-61 . 

Analcite  is  never  an  original  rock  constituent,  but  is  always  a 
product  of  secondary  secretion  or  alteration ;  in  the  first  case  it  occurs 


ANALC1TE—PEROFSKITE.  141 

in  cavities  as  freely  developed  or  attached  crystals  "with  the  forms 
oo  0  oo  .  2  0  2  (100)  (211),  or  2  O  2  (211),  or  else  it  completely  fills  the 
cavities  without  a  regular  crystal  form  ;  in  the  second  case  it  occurs  in 
pseudomorphs,  and  therefore  has  the  form  of  the  original  mineral,  con- 
sequently its  cross-sections  are  not  -characteristic.  The  most  frequent 
pseudomorphs  are  after  nepheline.  They  also  occur  after  leucite.  The 
cleavage  parallel  to  oo  0  oo  (100)  is  usually  quite  noticeable,  or  may 
be  developed  by  a  rapid  heating  of  the  section. 

The  index  of  refraction  is  low,  as  for  all  zeolites.  According  to 
Des  Cloizeaux,  np  =  1.4874.  Colorless  in  transmitted  light.  The  opti- 
cal anomalies,  which  are  very  common  in  the  freely  crystallized  anal- 
cites,  are  comparatively  rare  in  those  crystallized  in  mass,  if  the 
investigation  is  not  carried  on  under  a  sensitive  tone  of  color.  It  is 
highly  probable  that  the  double  refraction  is  a  result  of  internal  strain. 
A  gentle  warming  in  a  water  or  paraffine  bath,  according  to  A.  Meriam, 
diminishes  the  strength  of  the  double  refraction  very  considerably; 
and  according  to  C.  Klein  it  disappears  altogether  upon  stronger  heat- 
ing in  an  atmosphere  of  steam.  On  the  other  hand,  heating  to  red- 
ness, by  which  the  analcite  begins  to  lose  water,  increases  the  double 
refraction,  or  gives  rise  to  it  if  not  previously  present.  This  latter 
characteristic  may  be  used  in  the  diagnosis  of  analcite. 

Sp.  gr.  =  2.15-2.28.  Chemical  compositions  =  NaaO,  A12O3, 
4SiO2  -j-  2aq.  Soluble  in  all  mineral  acids  with  the  separation  of  gelat- 
inous silica ;  covered  with  a  thin  coat  of  hydrochloric  acid,  the  surface 
gelatinizes,  and  may  be  readily  colored.  The  clouding  due  to  the  loss 
o'f  water  when  heated  to  redness  is  very  marked,  often  complete. 

Analcite  with  strong  double  refraction  may  be  distinguished  from 
leucite,  which  it  closely  resembles,  by  its  gelatin ization  with  hydro- 
chloric acid  and  the  treatment  with  hydrofluosilicic  acid.  Analcite 
furnishes  almost  exclusively  the  characteristic  hexagonal  crystals  of 
sodium  fluosilicate ;  leucite,  a  preponderance  of  the  isometric  crystals  of 
potassium  fiuosilicate.  They  may  also  be  distinguished  by  their  specific 
gravities. 

Perofskite. 
Literature. 

A.  BEN  SAUDE,  Ueber  den  Perowskit.    Gottingen.  1882. 

EM.  BORICKY,  Ueber  Perowskit  als  mikroskopischen  Gemengtheil  ernes  fur  Bohmen 

neuen  Olivingesteins,  des  Nephelinpikrites.      Sitzungsber.  der  k.  bohni.  Ges. 

d.  Wissensch.  13.  Oktr.  1876. 
C.  KLEIN,  Perowskit  ^von  Pfitsch  in  Tyrol.    N.  J.  B.  1884.  I.  245-250. 


142  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

A.  SAUER,  Erlauterungen  zu  Section  Wiesenthal  der  geol.  Specialkarte  des  Konigr. 

Sachsen.     Leipzig.  1884.  54. 
A.  STELZNER,  Ueber  Melilith  und  Melilithbasalte.    N.  J.  B.  B.-B.  II.  1882.  390  ff. 

Perofskite  appears  in  the  eruptive  rocks  in  octahedrons  (PI.  XV. 
Fig.  2),  and  forms  microscopic  crystals,  0.02-0.03  mm.  in  diameter, 
which  are  usually  quite  sharp,  though  sometimes  rounded.  They  are 
occasionally  gathered  together  in  groups.  Incipient  forms  of  growth 
also  occur,  which  appear  like  intersection  twins  and  irregularly  ramify 
ing  skeleton  crystals ;  to  these  may  be  added  the  jagged  plates  men- 
tioned by  several  authors.  On  the  other  hand,  the  perofskite  crystals, 
which  occur  sparingly  in  the  Archaean  rocks  or  their  intercalations, 
almost  always  have  the  cubical  form. 

By  incident  light  perofskite  is  grayish  yellow  to  gray-brown,  the 
minute  crystals  appearing  like  fine  powder ;  in  transmitted  light  it  is 
grayish  white,  violet-gray,  gray-brown,  brownish  yellow  to  red-brown, 
seldom  with  greenish  tones,  more  rarely  with  a  slight  zonal  change  of 
color.  The  index  of  refraction  has  not  yet  been  measured  ;  it  is,  how- 
ever, very  strong,  and  over  l.Y.  Consequently,  the  total  reflection  is 
considerable,  and  the  surface  of  intersected  crystals  is  strongly  wrinkled. 
The  dark  borders  due  to  the  total  reflection  have  given  rise  to  numer- 
ous illusions  as  to  the  crystal  forms  and  the  presence  of  a  shelly  struc- 
ture. 

Perofskite  does  not  appear  isotropic  between  crossed  nicols,  but 
doubly  refracting,  so  that  in  the  larger  crystals  the  parts  with  different 
orientation  of  the  axes  of  elasticity  penetrate  one  another  in  the  form 
of  a  complicated  twinning,  with  optically  biaxial  striae  arranged  in  in- 
tersecting systems.  These  are  not  noticeable  in  very  small  crystals. 
The  above-cited  studies  of  C.  Klein  and  A.  Ben  Saude  render  it  highly 
probable  that  the  double  refraction  of  perofskite  is  an  anomaly,  and 
not  the  result  of  mimetic  structure. 

H.  =  5.5.  Sp.  gr.  =  4.1.  Chemical  compositions,  CaO,  TiO2.  Part 
of  the  lime  is  not  infrequently  replaced  by  FeO  in  considerable  quan- 
tity. Perofskite  is  not  attacked  by  hydrochloric  acid  nor  by  hydroflu- 
oric acid  in  water.  It  is  dissolved  by  concentrated  sulphuric  acid  upon 
being  heated.  Perofskite  is  easily  isolated  from  rocks  by  the  combined 
use  of  its  specific  gravity,  its  resistance  to  chemical  action,  and  its  very 
indifferent  behavior  towards  a  strong  electro-magnet. 

Perofskite  is  generally  quite  free  from  inclusions,  and  is  undecom- 
posed  in  rocks.  But  Sauer  (1.  c.)  observed  in  the  nepheline  basalt  of 
Oberwiesenthal  its  alteration  into  a  substance  (leucoxen)  quite  analo- 
gous to  the  alteration  product  of  ilmenite  and  rutile. 


PEROFSK1TE.  143 

Perof skite,  when  opaque,  is  easily  mistaken  for  the  iron  ores  and 
spinels ;  from  the  first  of  these  it  is  distinguished  by  the  lack  of 
metallic  lustre  and  its  insolubility  in  hydrochloric  and  nitric  acids. 

The  transparent  crystals  and  grains  of  perofsldte  are  easily  con- 
founded with  spinel,  chromite,  garnet,  and  titanite.  A  definite  deter- 
mination can  only  be  made  on  isolated  material  by  proving  the  pres- 
ence of  titanic  acid  and  the  absence  of  silica  and  chromium. 

Perofskite  is  an  almost  constant  ingredient  of  the  younger  basic 
eruptive  rocks,  especially  melilite  basalt.  It  occurs  in  leucite  and 
nepheline  rocks ;  more  rarely  in  elseolite  syenite  (Ditro).  It  occurs 
in  serpentinized  peridotite  in  the  Onondaga  salt  group  at  Syracuse, 
N.  Y.*  It  belongs  to  the  oldest  secretions  in  the  eruptive  magmas 
of  these  rocks,  and  therefore  occurs  as  inclusions  in  most  of  the  other 
constituents.  It  is  accompanied  by  magnetite  and  chromite,  with 
which  also  it  grows  together  and  often  surrounds  later  secretions  with 
a  kind  of  wreath. 

*  Geo.  H.  Williams,  Amer.  Journ.  Sci.  Vol.  XXXIV.  Aug.  1887. 


144         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


MINERALS  OF  THE  TETRAGONAL  SYSTEM. 

TETRAGONAL  minerals  are  doubly  refracting,  with  one  optic  axis.  The 
latter  coincides  with  the  principal  crystallographic  axis,  and  is  the  axis  of 
greatest  or  least  elasticit}'.  In  the  first  case,  the  character  of  the  double 
refraction  is  said  to  be  negative,  and  the  ordinary  ray  is  more  strongly 
refracted  (GO  >  e)  ;  in  the  second  case  the  substance  is  optically  posi- 
tive, and  the  extraordinary  ray  is  more  strongly  refracted  (GJ  <  e). 
Each  of  the  two  rays  is  differently  absorbed :  thus  tetragonal  minerals, 
when  colored,  exhibit  a  more  or  less  noticeable  pleochroism  in  all 
sections  which  do  not  lie  parallel  to  oP  (001).  Sections  at  right  angles 
to  c  have  quadratic  or  octagonal  outlines  or  cleavage  lamellae,  or  the 
regular  outline  is  wanting  and  the  cleavage  is  parallel  to  oP  (001) ; 
such  sections  behave  liko  isotropic  substances  in  parallel  polarized 
light — they  remain  dark  during  a  complete  rotation  between  crossed 
nicols;  in  convergent  polarized  light  they  show  an  interference  cross 
with  or  without  colored  rings,  whose  arms  lie  parallel  to  the  principal 
sections  of  the  nicols,  and  which  do  not  change  their  position  during  a 
rotation  of  the  section.  Sections  parallel  or  inclined  to  c  exhibit  out- 
lines varying  with  the  position  of  the  section  and  the  form  of  the  crys- 
tal ;  the  cleavage  lamellae  are  recognized  by  parallel  or  intersecting 
systems  of  cracks.  In  parallel  polarized  light  the  sections  are  doubly 
refracting ;"  for  a  complete  rotation  between  crossed  nicols  they  appear 
dark  and  light  four  times,  and  the  position  of  darkness  is  always 
reached  when  a  principal  section  of  the  nicols  is  either  parallel  to  the 
cleavage  cracks  or  bisects  the  angle  between  them.  In  convergent 
polarized  light  the  interference  figure  appears  at  one  side  of  the  field 
of  view  and  moves  around  the  margin  during  a  rotation  in  such  a 
manner  that  the  arms  of  the  cross  move  parallel  to  themselves,  so  long 
as  the  section  is  not  too  greatly  inclined  to  c ;  in  sections  parallel  to  c 
the  interference  figure  separates  into  hyperbolas  which  lie  symmetri- 
cally with  respect  to  the  principal  axis. 

Optical  anomalies  show  themselves  in  basal  sections  when  examined 
in  convergent  polarized  light  by  the  interference  cross  opening  into 
hyperbolas  during  a  rotation  of  the  section,  and  presenting  the  appear- 
ance of  a  biaxial  body  with  small  axial  angle,  cut  at  right  angles  to 
the  acute  bisectrix.  Different  portions  of  such  an  abnormal  plate 
usually  show  different  sizes  of  the  apparent  axial  angle,  and  a  varying 


RUTILE. 


145 


position  of  the  apparent  axial  plane.  In  parallel  polarized  light  the 
basal  section  sometimes  exhibits  a  division  of  the  field  into  irregularly 
lighted  parts. 


Literature. 

A.  CATHKEIN,  Ein  Beitrag  zur  Kenntniss  der  Wildschflnauer  Schiefer  und  der 

Thonschiefernadelchen.     K.  J.  B.  1881.  I.  169-183. 
A.  VON  LASAHLX,  Ueber  Mikrostruktur,  optisches  Verhalten  und  Umwandlung  des 

Rutil  in  Titaneisen.     Z.  X.  1883,  VIII.  59-75. 

A.  SAUER,  Rutil  als  mikroskopiscner  Gesteinsgemengtheil.     N.  J.  B.  1879.  569-576. 
—  Rutil  als  mikroskopischer  Gemengtheil  in  der  Gneiss-  und  Glimmerschiefer- 

formation,  sowie  als  Thonschiefernadelchen  in  der  Phyllitformation.     N.  J.  B. 

1881.  I.  227-238. 
L.  VAN  WERVEKE,  Rutil  im  Ottrelithschiefer  von  Ottrez  und  im  Wetzschiefer  der 

Ardennen.    N.  J.  B.  1880.  II.  281-283. 

Rutile  appears  in  rocks  under  a  great  variety  of  forms.  "Where  its 
dimensions  are  large  it  usually  takes  the  form  of  grains,  or  has  its  edges 
and  corners  rounded  ;  on  the  other  hand,  the  extremely  minute  micro- 
scopic individuals  in  certain  schists  possess  crystal  forms  of  almost  ideal 
sharpness.  Ill  :  111  =  84°  40'.  Their  habit  is  always  prismatic,  and 
the  forms  recognizable  are  the  same  as  on  the  macroscopic  crystals  ; 
they  also  possess  the  same  striation  parallel  c.  Twinning  is  extremely 
common,  and  follows  either  the  law,  the  twinning  plane  is  .Poo  (101)  ; 
or,  as  is  less  frequent  among  the  macroscopic  individuals,  the  twinning 


Fig. 


plane  is  3Poo  (301).     By  the  first  method  the  principal  axes  of  the 
twinned  individuals  form  an  angle  of  65°  35',  by  the  second  an  angle 
10 


146         PHYSIOGRAPHY  OF  THE  ROOK-MAKING  MINERALS. 

of  54°  44'.  Twins  of  the  first  kind  are  usually  genicnlar  (Fig.  54, 
ooP .  ooPoo  .  Poo  (110)  (100)  101)  ;  those  of  the  second  kind  are  heartl 
shaped  (Fig.  55,  ooP3 .  ooPoo  .Poo  .  P.  (310)  (100)  (101)  (111)). 
Not  infrequently  there  is  no  indication  of  twinning  in  the  outline  of 
the  larger  grains  and  crystals,  yet  in  polarized  light  they  prove  to  be 
twinned  through  and  through,  so  that  in  one  individual  a  greater  or 
less  number  of  lamellae  are  intercalated  in  twinned  position  after  the 
first-named  method.  These  lamellae  cut  the  principal  axis  either  at  an 
angle  of  65°  35'  or  57°  12.5',  and  stand  in  twinned  position  to  this 
or  to  themselves,  and  are  generally  arranged  parallel  to  all  faces  of 
Poo  (101).  In  a  section  parallel  to  ooPoo  (100)  this  lamination 
would  appear  as  in  Fig.  56 ;  in  a  basal  section  it  would  be  as  in  Fig.  57. 
The  lamellae  may  traverse  the  crystal  completely  or  only  in  part. 


O.  Miigge  has  shown  that  this  twinned  lamination  is  probably  a  result 
of  pressure,  the  face  Poo  (101)  serving  as  a  gliding-plane,  (cf.  N.  J.  B. 
1884,  I.  216.)  The  very  small  individuals  of  rutile  frequently  occur 
as  intergrown  twinned  needles,  and  form  net-shaped  groups,  called 
sagenite  by  De  Saussure,  represented  in  Fig.  58.  The  meshes  of  such 
a  sagenite  web  are  apparently  60°  and  120° ;  in  actual  fact  the  angles 
correspond  to  the  two  laws  just  given.  Such  structures  are  usually 
very  small,  the  length  of  the  single  individuals  being  -j-J-g-  to  1610o  mm. 
The  cleavage  of  rutile  parallel  ooP  (110)  is  very  perfect,  and 
shows  itself  as  fine  and  very  straight  cracks  (Figs.  56  and  57) ;  that 
parallel  ooPco  (100)  is  less  perfect :  the  cracks  are  rough,  irregular,  and 
frequently  interrupted ;  they  are  only  distinct  in  very  thin  sections. 
In  basal  sections,  therefore,  there  are  two  sj^stems  of  cracks  intersecting 
at  right  angles :  the  first  of  these  cuts  the  other  at  45° ;  in  sections 
inclined  to  the  principal  axis  the  cleavage  cracks  intersect  in  rhombic 


RUTILE.  147 

figures,  which  are  more  acute  as  the  inclination  is  less ;  in  sections 
parallel  to  c  all  the  cleavage  cracks  are  parallel.  f 

Kutile  is  strongly  refracting,  and  is  optically  positive ;  among  rock- 
making  minerals  there  are  none  with  higher  index  of  refraction  nor 
more  strongly  doubly  refracting.  Barwald  determined  on  rutile  from 
the  auriferous  sands  of  Syssert,  in  the  Urals, 

a>u  =  2.5671  eu  =  2.84:15. 

oo^  =  2.6158  ena  =  2.9029 

c»to  =  2.6725  ete  =  2.9817 

From  this  it  arises  that  the  surface  of  its  sections  is  very  distinctly 
wrinkled,  the  total  reflection  at  the  margin  is  strong,  and  the  inter- 
ference colors  even  of  the  minutest  needles  are  very  brilliant.  For  a 
thickness  of  hardly  -j-oVrr  min-  it  shows  red  of  the  first  order;  as  soon 
as  the  dimensions  are  somewhat  increased,  the  colors  are  the  indistinct 
ones  of  -a  higher  order,  which  are  no  longer  recognizable  when  the 
rutile  is  strongly  colored. 

The  pleochroism  is  changeable  in  those  rutiles  which  in  trans- 
mitted light  appear  yellow,  reddish  brown,  or  fox-red,  according  to  their 
thickness.  The  thicker  microscopic  crystals  and  the  extremely  minute 
needles  do  not  appear  at  all  pleochroic;  in  those  of  medium  size  O  is 
sometimes  yellow  to  brownish  yellow,  ^brownish  yellow  to  yellowish 
green,  the  absorption  \$E^>0.  In  consequence  of  this  pleochroism 
twinned  crystals  often  appear  to  be  colored  green  and  yellowish  in 
stripes. 

In  consequence  of  the  twinning  structure,  also,  basal  sections  of 
rutile  frequently  do  not  exhibit  the  simple  interference  figure  of  a 
uniaxial  mineral,  but  phenomena  which  suggest  more  or  less  strongly 
that  of  a  biaxial  crystal  cut  perpendicular  to  a  bisectrix.  Between 
crossed  nicols  the  section  is  dark  throughout  its  whole  extent  only  when 
the  diagonals  of  the  cleavage  parallel  ooP  (110)  coincide  with  the 
principal  sections  of  the  nicols.  In  all  other  positions  the  portions  of 
the  crystals  free  from  lamellae  remain  dark;  but  those  containing 
twinned  lamellae  are  variously  illumined  according  to  the  number  and 
position  of  those  lamellae,  as  may  be  seen  from  a  b  q  d 
Fig.  59,  which  represents  the  cross-section  of  a  p- 
basal  section  of  rutile  with  twinned  lamellae. 

The  high  sp.  gr.  =  4.20-4.25  of  rutile,  whose 
chemical  composition  is  TiO2,  and  its  resistance  to  hydrochloric  and  hy- 
drofluoric acids  facilitate  its  isolation  from  rocks  even  when  of  the  small- 


148         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

est  dimensions.  It  is  strongly  attacked  by  treatment  with  hydro- 
fluoric and  s/ilphuric  acids,  or  with  sulphuric  acid  alone.  To  distin- 
guish the  isolated  powder  from  zircon  and  cassiterite,  with  which  it 
may  be  confounded,  a  small  quantity  is  melted  to  a  bead  on  platinum 
wire  with  dehydrated  bisulphate  of  potassium,  and  this  is  tipped  with 
a  drop  of  hydrogen  superoxide.  "When  titanic  acid  is  present  the  bead 
and  the  liquid  are  colored  yellow  or  orange,  according  to  the  amount 
of  titanium  present. 

Rutile  is  frequently  found  altered  into  a  fibrous  or  granular  sub- 
stance, strongly  refracting,  of  white,  yellowish  or  greenish  color  (PL 
XY.  Fig.  3),  which  is  identical  with  leucoxene,  an  alteration  product 
of  ilmenite.  It  has  been  called  titanomorphite  by  von  Lasaulx,  and 
wrongly  supposed  to  be  calcium  bititanate.  Sauer  has  taken  the  same 
substance  in  similar  rocks  for  titanic  acid.  Cathrein  proved  it  to  be 
titanite.  Rutile  is  also  altered  into  ilmenite. 

Untile  may  also  occur  as  a  secondary  product;  it  has  been  found  as 
the  alteration  of  titanite  in  an  elseolite  syenite  of  the  Serra  de  Mon- 
chique,  and  as  an  alteration  product  from  ilmenite  in  altered  diabase.* 
It  is  probable  that  the  sagenite  webs  found  in  the  decomposed  micas  of 
the  kersantites  and  minettes,  and  those  in  altered  phlogophites,  are 
products  of  the  leeching  out  of  the  minerals  containing  them.  The 
rutile  needles  often  lie  in  three  directions,  intersecting  at  angles  of 
60°,  and  parallel  to  the  rays  of  the  pressure  figure.  Rutile  is  also  a 
secondary  product  in  the  hornblendes  of  many  diorites. 

Rutile  occurs  as  a  primary  constituent  both  in  eruptive  rocks  and 
in  the  schists,  but  more  frequently  in  the  latter.  G.  W.  Hawes  f  con- 
sidered the  long  hair-like  interpositions  widely  disseminated  in  the 
quartzes  of  granite  as  rutile,  although  he  was  not  able  to  definitely  de- 
termine them,  their  breadth  being  so  minute  that  they  appear  opaque. 

M.  Maclay-Miklucko  J  isolated  and  measured  rutile  crystals  from 
the  mica  of  the  topaz-bearing  granite  of  Greif  en  stein,  which  were 
accompanied  by  microscopic  cassiterite. 

G.  H.  Williams  §  determined  rutile  as  interpositions  in  the  mica  of  a 
porphyritic  diorite  out  of  the  gneiss  from  the  region  of  Triberg,  in  the 
Black  Forest.  K.  A.  Lossen  (1.  c.)  discovered  it  as  a  primary  constitu- 
ent in  peculiar  concretionary  mineral  masses,  which  occur  as  inclusions 
or  like  older  secretions  in  the  kersantite  of  Michaelstein  in  the  Hartz. 

*  G.  H.  Williams,  N.  J.  B.    1887.  II.  263. 

f  Mineralogy  and  Lithology  of  New  Hampshire.     Concord.     1878.    45. 

%  N.  J.  B.     1885.  II. 

§  K  J.  B.  B.-B.     II.  1883.  617. 


ANATASE.  149 

Rutile  is  very  common  in  grains  and  crystals  in  the  gneisses  and 
mica  schists  and  the  masses  intercalated  in  them,  especially  in  the  rocks 
rich  in  hornblende  and  augite.  It  is  very  widely  disseminated  in  the 
phyllite  formation.  And  it  is  in  the  phyllitic  slates  that  the  sagenite 
webs  are  most  perfectly  developed: 

The  extremely  minute  microlitic  needles  which  F.  Zirkel  first  called 
attention  to  in  the  clay  slates  and  roofing  slates,  and  which  have  been 
known  as  clay-slate  needles  (thonschiefernadeln)  (PI.  XY.  Fig.  4),  were 
shown  by  A.  Cathrein  to  be  rutile.  For  further  notes  on  the  distri- 
bution of  rutile  the  student  should  consult  the  work  of  H.  Thiirach.* 

In  all  rocks  rutile  is '  characterized  by  the  more  or  less  complete 
absence  of  interpositions. 

Anatase. 
, .  Literature. 

J.    S.   DILLER,  Anatas  als  Umwandlungsprodukt  von  Titanit  im  Biotitamphibol- 

granit  der  Troas.     N.  J.  B.  1883.  I.  187-193. 
A.  STELZNER,  Studien  liber  Freiberger  Gneisse  und  ihre  Verwitterungsprodukte. 

N.  J.  B.  1884.  I.  271-274. , 
H.  THURACH,  Ueber  das  Vorkommen  mikroskopischer  Zirkone  und  Titanmineralien 

in  den  Gesteinen.     Wurzburg.  1884. 

Anatase  always  occurs  in  crystals,  and  is  never  massive  ;  its  habit  is 
predominantly  pyramidal,  more  rarely  tabular,  and  never  prismatic. 
Of  the  pyramidal  forms,  which  are  known  macroscopically,  a  con- 
siderable number  also  occur  in  the  microscopic  crystals.  P  (111), 
however,  predominates,  and  determines  the  habits  of  the  crystals, 

111  :  111  —  136°  36'.     Less  frequently  a  pyramid  —  Poo,  not  yet 

tin> 

•completely  identified,  or  the  base  determines  the  type  of  the  crys- 
tals.— The  fundamental  pyramid  is  always  striated  parallel  to  the  edge 
P  :  oP  (111)  :  (001). — On  account  of  the  minute  dimensions  of  the 
crystals  they  are  generally  seen  microscopically  as  complete  bodies ; 
their  outlines  in  cross-sections  are  quadratic  parallel  to  oP,  acutely 
rhombic  parallel  to  c. 

The  cleavage  parallel  P  (111)  and  oP  (001)  is  sharp  and  clear  in 
cross  sections,  but  is  not  noticeable  when  the  crystals  are  not  cut  by 
the  surfaces  of  the  thin  section. 


*  Ueber  das  Vorkommen  mikroskopischer  Zirkone  und  Titan  mineralien  in  den 
<Gesteinen.     Wurzburg.  1884. 


150        PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

Schrauf  *  determined 

GOB  =  2.511  and  2.515  eB  =  2.476  and  2.477 

(*>D  =  2.534  eD  =  2.496  and  2.497 
Miller  found 

GO    =  2.554  e    =  2.493 

Among  rock-making  minerals  rutile  alone  has  a  higher  index  of 
refraction  ;  the  double  refraction  in  anatase  is  lower  than  in  dolomite 
and  calcite,  but  somewhat  higher  or  at  least  as  high  as  in  zircon.  The 
crystal  form  is  seldom  easily  recognized  because  of  the  strong  total 
reflection  along  the  margin  of  the  crystals ;  the  relief  is  strong,  and  the 
interference  colors,  even  for  very  small  thicknesses,  are  of  the  second 
and  third  order.  With  somewhat  thicker  crystals  the  colors  are  those 
of  the  fourth  and  fifth  order,  which  approach  white.  In  basal  sections 
the  interference  cross  is  accompanied  by  several  rings.  The  character 
of  the  double  refraction  is  negative.  The  pleochroism  generally  is 
not  strong,  and  varies  considerably  with  the  color:  in  blue  crystals  the 
color  for  ^is  deep  blue,  for  0  light  blue  ;  in  yellow  crystals  j^is  light 
yellow,  0  orange,  according  to  von  Lasaulx. 

Optical  anomalies,  which  show  themselves  by  causing  the  interfer- 
ence cross  to  separate  into  hyperbolas  during  a  rotation  between  crossed 
nicols,  have  led  Mallard  to  consider  anatase  mimetic.  The  interference 
cross  of  the  blue  crystals  is  not  black,  but  apparently  blue. 

Anatase  exhibits  an  adamantine  lustre  by  incident  light ;  by  trans- 
mitted light  it  is  at  times  colorless,  or  yellow  of  different  intensity,  or, 
brown ;  at  times,  blue  in  different  shades ;  seldom  green.  The  color 
often  varies  in  one  crystal,  either  parallel  to  the  pyramidal  cleavage 
cracks  in  concentric  bands,  or  parallel  to  the  diagonals  of  this  cleavage. 
The  colorless  or  yellow  portions  usually  behave  optically  normal,  while 
the  blue  portions  appear  more  often  to  exhibit  anomalies. 

Sp.  gr.  =  3.9.  Chemical  composition  =  TiO2.  Its  chemical  behav- 
ior is  the  same  as  that  of  rutile,  from  which  it  is  easily  distinguished 
by  the  form,  cleavage,  and  optical  character.  Anatase  is  usually  free 
from  interpositions. 

Anatase  has  not  yet  been  observed  as  a  primary  constituent  of 
rocks.  In  all  its  occurrences  it  must  be  considered  an  alteration  prod- 
uct of  titaniferous  minerals.  Thus  Diller  found  it  as  a  probable  al- 
teration product  of  titanite  in  the  hornblende  biotite  granite  of  the 

*  S.  W.  A.  XLII.  1860. 


CASSITER1TE.  151 

Troad,  and  of  ilmenite  in  the  Schalstein  of  Redwitz,  near  Hof,  Bavaria. 
It  has  been  found  in  gneiss,  diabase,  quartz  porphyry.  Thiirach  observed 
it  in  various  granites,  diorites,  and  crystalline  schists  ;  also  in  numerous 
grauwackes,  sandstones,  shales  (Schiefer  thonen)  and  limestones  of  all 
formations,  from  the  Silurian  to  the  Tertiary. 

Anatase  in  some  cases  is  probably  derived  from  rutile,  as  the  latter 
is  sometimes  an  alteration  product  of  anatase.  Titanic  iron  is  also 
found  among  the  cleavage  cracks  of  the  blue  Brazilian  anatase,  in  the 
same  manner  as  was  described  for  rutile. 

Cassiterite. 

Cassiterite  forms  short  prismatic  or  pyramidal  crystals  and  twins; 
or  its  crystallographic  boundary  may  be  entirely  wanting.  Ill :  111  =  87° 
T '.  Twinning  is  not  so  general  as  for  rutile,  with  which  otherwise  the 
cross-sections  and  their  angles  correspond  closely.  It  sometimes  occurs 
in  radially  columnar  aggregates ;  the  single  individuals  are  then  long, 
slender  prisms. 

The  cleavage  parallel  ooPoo  (100)  is  not  noticeable,  or  at  most  is 
only  in  traces,  on  the  microscopic  individuals  and  on  the  cross  sections ; 
this  is  one  of  the  most  important  means  of  distinction  from  rutile. 

Optically  positive.  Index  of  refraction  high.  Double  refraction 
strong ;  consequently  the  interference  colors  are  only  recognizable  on 
very  thin  lamellae.  Grubemann  has  determined  on  the  Cassiterite  of 
Schlaggenwalde : 

For  the  red  part  of  the  spectrum  GO  =  1.9793  e  =  2.0799 
For  the  yellow  part  of  the  spectrum  GO  =  1.9966  e  =  2.0934 
For  the  green  part  of  the  spectrum  GO  =  2.0115  e  =  2.1083 

In  transmitted  light  yellowish  to  brown  or  red  in  different  shades, 
seldom  almost  colorless,  often  variously  colored  in  bands  and  stripes ; 
by  incident  light  almost  metallic  adamantine  lustre.  The  interference 
cross  not  infrequently  separates  into  hyperbolas  upon  rotation. 

Sp.  gr.  =  6.87.  Chemical  composition  =  SnO2.  Unattacked  by 
acids.  Distinguished  from  rutile  and  zircon  with  certainty  only  by 
means  of  its  specific  gravity,  measurements  of  angles  on  isolated  crys- 
tals, or  by  its  chemical  reaction.  The  coloring  ruby-red  of  a  borax 
bead,  previously  colored  blue  by  copper  vitriol,  may  be  accomplished 
after  sufficient  practice  even  with  extremely  minute  particles. 

Up  to  the  present  time  Cassiterite  has  only  once  been  unquestiona- 
bly shown  to  exist  as  a  microscopic  rock  constituent,  and  then  it  oc- 


152 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


curred  with  rutile  as  inclusions  in  the  lithia  mica  of  the  granite  of  Grei- 
fenstein.*  It  appears  to  be  locally  abundant  in  gneisses  and  granites 
specially  those  of  Cornwall,  and  in  the  contact  zones  of  the  schists  in 
the  immediate  vicinity  of  the  granites.  It  can  only  be  definitely  de- 
termined by  isolating  the  crystals  from  these  rocks. 

Zircon. 
Literature. 

K.  VON  CHRTJSTSCHOFF,  Beitrag  zur  Kenntniss  der  Zirkone  in  Gesteinen.     T.  M. 
P.  M.  1886.  VII.  423. 

H.  ROSENBUSCH,  Sulla  presenza  dello  zircone  nelle  roccie.     Atti  della  R   Accad. 
delle  Sc.  Torino.  XVI.  1881. 

A.  E.  TORNEBOHM,  Om  Zirkonens  utbredning  in  bergarterne.     Geol.  For.  i  Stock- 
holm Forhandl.  1876.  III.  No.  34. 

TH.  VON  UNGERN-STERNBERG,  Untersuchungen  uber  den   Finnlandischen  Rapa- 
kiwi-Granit,     Inaug.-Diss.  Leipzig.  1882. 

Zircon  occurs  as  a  primary  constituent  of  rocks  only  in  the  form 
of  crystals,  never  massive ;  111 :  111  =  84°  20'.  The  habit  of  the  micro- 
scopic crystals  is  almost  always  short  prismatic,  seldom  long,  and  very 
rarely  pyramidal.  The  forms  are  the  same  as  those  found  on  the  mac- 
roscopic crystals.  It  is  to  be  remarked  that  the  pyramid  3P3  (311) 

is  very  often  the  predominant  form 
on  the  poles  of  the  principal  axis  of 
the  microscopic  crystals,  and  that  not 
infrequently  other  biquadratic  pyramids 
occur.  Fig.  60  shows  some  of  the 
rarer  forms;  PL  XY.  Fig.  5  shows 
some  of  the  commonest.  The  form  of 
the  cross-sections  is  readily  derived  from 
a  consideration  of  the  crystal  forms. 
The  crystals  are  seldom  shorter  than  0.01 
mm.  Twinning  has  not  yet  been  noted. 
The  imperfect  cleavage  parallel  to  the  prism  and  pyramid  is  not 
noticeable  microscopically  on  the  crystals,  and  but  rarely  on  cross-sec- 
tions; on  the  other  hand,  in  basal  sections  of  large  individuals  the 
cleavage  parallel  to  coP  (110)  is  very  distinct,  and  that  parallel  to 
ooPoo  (100)  is  observed  in  traces.  One  must  be  careful  not  to  mis- 
take the  shell-like  (zonal)  structure,  so  common  in  this  mineral,  for 
cleavage. 

The  index  of  refraction  of  zircon  and  its  double  refraction  are  both 
very  strong;    its  optical  character  is  positive.     On  the  hyacinth  of 
Ceylon  the  following  constants  have  been  determined  :  GJ  =  1.960,  e  = 
*  M.  Maclay-Miklucko.     K  J.  B.  1885.  II.  88~ 


GO 


ZIRCON.  153 

2.015  (Brewster),  c»p=:1.92,  ep  =  1.9T  (Senarmont),  G?^  =  1.9239, 
ena=  1.9682  (Sanger),  and  on  the  zircon  from  Miask,Urals,  cona  =  1.9313, 
ena  —  1.9931  (Sanger).  These  numbers  explain  the  high  relief,  the  broad 
total  reflection  borders,  the  wrinkled  surface  of  cross-sections  and  the 
brilliant  red  and  green  interference  colors  with  crossed  nicols,  which 
are  exhibited  by  the  smallest  individuals.  Sections  parallel  to  the  base 
oP  (OjOl)  yield  several  rings  about  the  dark  cross  in  convergent  light. 
Here  and  there  in  zircon,  also,  the  interference  cross  separates  into  hy- 
perbolas, especially  when  the  shelly  structure  is  quite  strongly  devel- 
oped. 

A  pleochroism  which  is  often  very  distinct  in  macroscopic  crystals 
is  very  faint  in  the  microscopical  occurrences,  and  is  more  generally 
not  noticeable  at  all.  Haidinger  observed  in  the  brownish  pearl-gray 
crystals  of  Ceylon  :  O,  clove-brown  ;  E,  asparagus-green  ;  in  light 
clove  brown  crystals,  <?,  grayish  violet-blue ;  E,  grayish  olive-green ; 
in  yellowish  white  crystals  from  the  same  locality,  0,  light  blue  ;  jEJ 
light  yellow. 

The  microscopic  zircons  are  mostly  colorless  or  very  light  yellow 
and  pink  or  violet ;  very  seldom  reddish  to  brownish  from  a  coating  of 
limonite,  which  may  be  removed  with  hydrochloric  acid.  "When  color 
is  present  it  is  not  usually  uniformly  distributed,  but  is  arranged  in 
zones,  or  at  the  centre  or  along  the  axes  of  the  crystals.  Shelly  or 
zonal  structure  is  very  common  ;  it  generally  repeats  the  outward  form 
of  the  crystal,  when  this  is  made  up  of  ocP  .  P  .  (110)  (111).  It  rarely 
indicates  different  forms  from  those  developed  on  the  crystal.  How- 
ever, it  frequently  happens  that  the  lines  of  the  zones  are  straight  so 
long  as  they  follow  the  prisms,  but  appear  rounded  at  the  poles  of  the 
crystals. 

Inclusions  in  zircon  are  not  infrequent,  but  are  difficult  to  deter- 
mine on  account  of  the  high  index  of  refraction  of  their  matrix.  Of 
the  unindividualized  interpositions,  fluid  ones  may  be  recognized  with 
certainty  by  the  movement  of  their  bubbles ;  inclusions  without  bub- 
bles, which  always  have  very  dark  borders  and  occur  in  various  forms, 
may  be  either  gas  or  glass  inclusions.  Indeterminable  acicular  micro- 
lites  also  occur. 

H.  =  7.5.  Sp.  gr. =4.4-4.7.  Chemical  composition  =  ZrO2,  SiO2. 
Not  appreciably  attacked  by  acids;  it  is  easily  isolated  from  rocks  on 
account  of  its  specific  gravity,  its  resistance  to  acids,  and  its  indifference 
to  magnetic  attraction.  The  crystals,  then,  though  of  very  minute  di- 
mensions, may  be  measured  by  a  goniometer.  As  a  more  certain  test, 
a  small  portion  may  be  fused  with  bicarbonate  of  soda  in  a  platinum 


154         PHYSIOGRAPHY  OF  THE  ROCK-MAKISG  MINERALS. 

dish  in  the  proportion  of  1 : 4  to  1 : 10,  in  order  to  obtain  hexagonal 
plates  of  ZrO2. 

Zircon  occurs  very  constantly  and  often  abundantly  in  the  acid 
eruptive  rocks  of  the  granite  series,  syenite,  diorite,  and  gabbro,  as 
well  as  in  their  porphyritic  equivalents ;  it  occurs  less  abundantly,  but 
quite  constantly,  in  the  more  basic  eruptive  rocks  of  the  diabase  family, 
and  in  all  the  younger  eruptive  rocks.  It  is  also  a  constant  and  often 
an  abundant  constituent  of  Archaean  rocks,  especially  of  the  gneisses. 
Formerly  it  was  very  generally  mistaken  for  rutile.  Zircon  was  recog- 
nized by  G.  W.  Hawes*  in  granites. 

A.  E.  Tornebohm  was  the  first  to  comprehend  the  wide  distribution 
and  petrographical  importance  of  zircon.  It  has  been  isolated  from  the 
rapakiwi  of  Finland,  and  determined  qualitatively  and  goniometrically 
by  H.  Kosenbusch  and  Th.  von  Ungern-Sternberg.  Its  distribution 
in  different  eruptive  and  schistose  rocks  has  been  noted  by  H.  Thii- 
rach.f  It  is  found  in  the  sand  derived  from  granites  and  Archaean 
rocks ;  and  also  as  a  foreign  constituent  in  fossiliferous  rocks,  lime- 
stones, shales,  marls,  sandstones,  etc. 

In  all  cases  zircon  belongs  to  the  oldest  constituents  of  the  rocks  it 
occurs  in  ;  consequently  its  crystal  form  is  always  perfect,  and  it  may  be 
enclosed  in  any  of  the  other  minerals  associated  with  it.  It  is  un- 
doubtedly older  than  all  the  silicates,  but  its  relative  age  as  compared 
with  apatite  and  the  ores  is  not  so  certain.  When  enclosed  in  mica, 
pyroxene,  hornblende,  cordierite,  etc.,  it  is  often  surrounded  by  a 
pleochroic  halo.:): 

Scapolite  Group. 
Literature. 

FR.  BECKE,  Die  Gneissformation  des  niederOsterreichischen  Waldviertels.     T.  M.  P, 

M.  1882.  IV,  369  et  passim. 
V.  GOLDSCHMIDT,  Ueber  Verwendbarkeit  einer  Kaliumquecksilberjodid-Losung  bei 

mineralogischen  und  petrographischen  Untersuchungen.     N.  J.  B.  B.-B.  1.  p. 

225  fl.  1880. 
A.  MICHEL-LEVY,  Sur  une  roche  a  sphene,  amphibole  et  wernerite  granulitique  des 

mines  d'apatite  de  Bamle,  pres  Brevig  (Norvege).     Bull.  Soc.  min.  Fr.  1878.  I. 

43-46. 

*  Mineralogy  and  Lithology  of  New  Hampshire.     Concord.  1878.  75. 

f  Ueber  das  Vorkommen  mikroskopischer  zirkone  und  Titan  mineralien  in  den 
Gesteinen.  Wurzburg.  1884. 

|  A.  Michel  Levy,  "Sur  les  noyaux  a  polychroisme  intense  du  mica  noir."  C. 
R.  1882.  XCIV.  Hj.  Gylling,  Nagra  ord  om  Rutile  och  Zirkon,  etc.  Geol.  Foren. 
i  Stockholm  Forhdl.  1882.  VI.  No.  74.  162.  sqq.  G.  H.  Williams,  N.  J.  B.  B.-B. 
II.  1883. 


SCAPOLITE  GROUP.  155 

HJ.  SJOGREN,  Om  de  norska  apatitforekomsterna  och  sannolikheten  att  antraffa 
apatit  i  Sverige.  Geol.  Foren.  i  Stockholm  Forhandl.  1882.  VI.  No.  81.  469  ff. 

A.  E.  TOKNEBOHM,  Ett  par  skapolitforande  bergarter.  Geol.  Foren.  i  Stockholm 
FSrhdl.  1882.  VI.  No.  75.  193  ff. 

G.  TSCHERMAK,  Die  Skapolithreihe.     S.  W.  A.  LXXXVIII.  Nov.  1883. 

The  rock-making  sea/polite  minerals  usually  show  no  outward 
crystal  shape,  but  form  irregularly  defined  grains,  or  fibrous  aggregates, 
mostly  with  a  confused  arrangement.  Except  those  which  occur  in 
limestone,  whose  outline  in  the  prism  zone  is  generally  formed 
by  ooP  (110),  ooPoo  (100)  rarely  <x>P2  (210)  or  ooP3  (310), 
and  much  more  rarely  by  the  pyramidal  termination  P  (111). 
Ill  :  111  =  63°  42'.  Consequently  for  aggregates  the  sections  are 
irregular,  but  for  imbedded  crystals  they  are  quadratic  or  octagonal  in 
cross- sections,  and  rectangular  or  long  lath-shaped  in  longitudinal  sec- 
tions. 

The  cleavage  parallel  to  ooPoo  (100)  is  recognized  by  distinct 
parallel  cracks  in  longitudinal  sections,  and  by  rectangularly  intersect- 
ing ones  in  cross-sections  (PL  IX.  Fig.  6) ;  it  is  generally  more  notice- 
able in  those  occurrences  which  are  no  longer  entirely  fresh.  A  trans- 
verse parting  by  which  the  prisms  separate  into  several  members  is 
very  common  to  the  lath-shaped  individuals,  and  plays  an  important 
part  in  the  process  of  alteration  of  the  crystals.  H.  —  5.5. 

In  a  fresh  condition  the  scapolite  minerals  are  colorless  and  trans- 
parent, more  rarely  gray  to  brown  because  of  needle-shaped  interposi- 
tions which  are  as  yet  indeterminable.  All  scapolites  are  optically 
negative,  with  an  index  of  refraction  which  is  not  high,  but  with  strong 
double  refraction.  There  has  been  determined,  on  Yesuvian  meionite, 

cona  =  1.594-1.597  ena  =  1.558-1.561  (Des  Cloizeaux) ; 

on  dipyre  from  Pouzac, 

cona  —  1.5673  ena  =  1.5416  (Lattermann) ; 

cop    =  1.558  €p    =  1.543  (Des  Cloizeaux); 

on  scapolite  from  Arendal,  Norway, 

Goft    —  1.566  ep    —  1.545  (Des  Cloizeaux). 

The  index  of  refraction  apparently  sinks  with  a  decrease  of  the  Ca 
percentage,  and  an  increase  of  the  alkali  percentage.  The  interference 
colors  in  longitudinal  sections  in  consequence  of  the  great  difference, 
GO—  e,  are  more  brilliant  than  for  most  of  the  colorless  minerals, 
especially  for  the  feldspars  and  feldspar-like  substances,  as  well  as  for 
quartz ;  even  in  very  thin  sections  they  seldom  fall  below  orange  and 
yellow  of  the  1st  order.  Cross-sections  in  convergent  light  yield  a 
distinct  cross,  and  with  sufficient  convergence  the  first  ring  may  be 


156         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

plainly  seen.  On  the  other  hand,  the  relief  is  slight,  and  sections  iv. 
Canada  balsam  show  no  roughened  surface. 

Scapolite  is  distinguished  from  feldspar  and  cordierite  by  its 
uniaxial  character  and  the  cleavage ;  from  quartz  by  the  cleavage  and 
the  character  of  the  double  refraction ;  from  apatite  by  the  index  of 
refraction,  the  cleavage,  and  the  chemical  reaction  with  phosphoric  acid. 

The  specific  gravity  rises  with  the  percentage  of  lime  from  2.569 
in  marialite  to  2.735  in  meionite.  According  to  Tschermak's  investi- 
gations, the  scapolite  group  presents  a  continuous  series  of  isomorphous 
mixtures  of  two  silicates,  which  are  not  known  to  occur  by  themselves. 
One  of  them,  8CaO,  6A12O3,  12SiO2  =  Si12Al12Ca8O60,  predominates 
in  meionite  at  88  per  cent,  and  is  called  the  meionite  molecule  =  Me  • 
the  other,  3Na2O,  3A12O3,  18SiO2  +  2NaCl  =  Si1BAl.Na8O48Cla,  occurs 
in  marialite  at  84  per  cent,  and  is  called  the  marialite  molecule  =  Ma. 
The  less  siliceous  mixtures  of  Me  to  Me2Maj  are  completely  decom- 
posed by  acids,  or  nearly  so,  without  gelatinization ;  the  mixtures  of 
Me2Maj  to  MejMa.,  are  only  slightly  attacked  by  acids,  and  the 
more  acid  mixtures  from  MejMa,,  to  Me0Ma,  completely  resist  the 
attack  of  acids. 

The  rock-making  scapolite  minerals  have  not  as  yet  been  sufficiently 
investigated  chemically  to  refer  them  with  certainty  to  one  of  these 
three  groups,  but  it  is  the  mixtures  containing  a  medium  and  higher 
percentage  of  silica  which  appear  especially  widespread. 

There  are  no  inclusions  which  particularly  characterize  the  minerals 
of  this  family  ;  besides  the  minerals  associated  with  them  in  the  rocks, 
especially  epidote,  calcite,  diopside,  actinolite,  magnetite,  pyrite,  and 
feldspar,  there  are  fluid  inclusions  of  irregular  shapes  or  in  negative 
crystal  forms.  If  the  scapolites  have  been  formed  epigenetically  from 
other  minerals  (feldspar)  they  sometimes  contain  the  interpositions  of 
the  parent  mineral. 

The  scapolites  withstand  but  slightly  the  action  of  the  atmosphere 
and  of  surface  waters;  altering  easily  from  the  cross  fractures  and 
cleavage  cracks  into  a  fibrous  substance,  not  yet  definitely  determined, 
but  not  unlike  zoisite  from  its  low  double  refraction,  or  into  a  lamellar 
aggregation  of  kaolin  or  muscovite.  It  also  weathers  into  carbonates. 

Meionite  forms  attached  crystals,  and  does  not  appear  to  occur  as 
.an  actual  rock-making  mineral. 

With  the  exception  of  dipyre,  all  the  rock-making  minerals  of  the 
scapolite  group  are  here  designated  as  scapolites.  They  never  occur  as 
primary  constituents  in  eruptive  rocks,  but  are  sometimes  developed  in 
them  at  the  expense  of  the  feldspars.  As  primary  minerals  they  abound 


VESUVIANITE.  157 

in  the  Archaean  rocks,  where  they  occur  not  only  in  the  limestone 
layers,  but  also  as  constituents  of  the  gneisses,  especially  those  rich  in 
lime,  and  in  the  epidote  and  pyroxene-bearing  varieties.  Such  occur- 
rences are  treated  of  in  the  works  of  Becke  and  Tornebohm,  cited 
above. 

Dipyre  and  couzeranite  belong  to  the  scapolites  in  which  the 
marialite  molecule  predominates.  Both  minerals  are  to  be  considered 
as  identical,  the  difference  arising  mainly  from  the  want  of  pure^ 
material  for  the  investigation  of  the  second  variety.  Dipyre  accom- 
panies the  contact  metamorphism  of  limestone  and  schists  in  the 
Pyrenees.  In  granular  limestone  it  is  usually  well  crystallized  in  the 
vertical  zone,  while  in  the  schists  it  furnishes  irregularly  rounded  and 
elliptical  sections.  In  limestone  it  is  quite  free  from  inclusions,  with 
the  exception  of  calcite.  In  the  schists  it  is  often  completely  filled 
with  carbonaceous  particles,  muscovite  plates,  rutile  needles,  and  quartz 
grains;  and  by  incident  light  and  even  in  transmitted  light  with  a  low 
magnifying  power  it  appears  yellowish,  reddish,  bluish,  or  almost 
opaque.  These  inclusions  are  still  more  abundant  in  couzeranite, 
which  also  forms  irregular  grains,  or  quadratic,  sometimes  octagonal 
(00  P  .  cnP  oo)  prisms  without  terminal  faces.  It  is  easily  confused  with 
andalusite,  which  may  be  avoided  by  observing  the  cross-section  in 
convergent  polarized  light.  It  is  also  a  contact  mineral  in  limestones 
and  schists.  The  microscopical  characteristics  of  'the  dipyres  have 
been  given  by  Zirkel,*  Fischer,  f  and  v.  Lasaulx.J 

Vesuvianite. 

The  rock-making  vesuvianite  occurs  more  frequently  in  irregular 
pieces  or  prismatic  aggregates  than  in  crystals;  well-crystallized 
individuals  only  occur  in  granular  limestone,  and  then  have  the  faces 
&P  .  oo^oo  (110)  (100)  in  the  prism  zone ;  as  terminal  faces  oP 
(001)  appears  to  predominate,  jP(lll)  to  be  subordinate.  Ill  :  111 
=  74°  27'. 

The  imperfect  cleavage  parallel  to  the  prism  faces  is  indicated 
microscopically  by  a  few  cracks,  usually  short ;  they  only  follow  one 
prism,  probably  ooPoo  (100).  H.  --=  6.5. 

By  transmitted  light  vesuvianite  is  nearly  colorless,  yellowish  to 
greenish  yellow,  rose-red,  very  seldom  dark  reddish  brown  or 
blue.  The  colors  often  vary  in  concentric  shells  or  more  rarely  in 

*  Z.  D.  G.  G.  1867.  XIX.  202. 

f  Kritische,  mikroskop-mineralog.  Studien.  I.  Fortsetzung,  Freiburg  i.  B.  1871.  52. 
N.  J.  B.  1872.  848. 


158         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

irregular  patches,  especially  toward  the  centre  of  the  crystal.  The  index 
of  refraction  is  high  (on  a  crystal  from  Ala  it  was  nna  —  1.7258),  the 
surface  of  the  section  is  therefore  rough.  The  double  refraction  is 
very  small,  with  negative  character,  the  difference  GO  —  e  scarcely 
exceeding  0.0015. 
On  idocrase  from  Ala,  Tyrol, 

cona  =  1.719-1.722  ena  =  1.718-1.720  (Des  Cloizeaux) ; 

consequently  the  interference  colors  are  very  low.  The  double  refrac- 
tion varies  in  intensity  in  one  and  the  same  crystal,  sometimes  in 
concentric  zones ;  therefore  there  may  be  stripes  of  different  interference 
colors  in  the  crystal  between  crossed  nicols,  though  it  is  of  absolutely 
uniform  color  in  ordinary  light.  Moreover,  the  character  of  the  double 
refraction  may  change  in  the  different-colored  stripes,  so  that  with 
predominating  optically  negative  stripes  there  may  occur  optically 
positive  ones  (Hammrefjeld  in  Norway).  The  extinction  in  longi- 
tudinal sections  remains  parallel  to  the  prism  edges  for  all  the  stripes. 
In  the  cross-sections  in  thin  section  the  interference  cross  is  very 
faintly  seen  without  the  slightest  trace  of  rings.  The  pleochroism  is 
generally  weak  to  very  weak ;  the  ordinary  ray  colorless  or  yellowish  ; 
the  extraordinary,  reddish,  yellowish,  or  greenish.  Yesuvianite  may  be 
easily  confounded  with  pistacite  on  account  of  its  pleochroism  and 
index  of  refraction ;  but  the  small  double  refraction  is  a  sure  means  of 
distinction  without  considering  the  cleavage,  position  of  the  axes  of 
elasticity  and  the  phenomena  in  convergent  light. 

Sp.  gr.  =  3.40-3.47.  Yesuvianite  is  a  lime-alumina  silicate,  whose 
formula  is  not  exactly  known  :  it  contains  some  water  among  its  bases, 
and  in  certain  occurrences  fluorine,  according  to  Jannasch.  Besides 
CaO  it  contains  MgO  and  MnO;  besides  A12O3,  Fe2O3,  and  also  small 
quantities  of  alkalies.  It  is  not  unattacked  by  acids;  it  fuses  easily 
with  intumescence  to  green  or  brown  glass,  which  is  then  soluble  with- 
out difficulty  in  HC1  with  the  separation  of  SiO2. 

Yesuvianite  occurs  principally,  if  not  exclusively,  in  metarnorphic 
rocks;  it  is  widely  disseminated  in  the  limestones  and  lime-silicate 
hornstones  of  many  granite  contact  zones. 

It  frequently  forms  porphyritic  crystals  in  the  granular  limestones 
of  the  Archsean,  as  well  as  a  constituent  of  the  closely  related 
inclusions  of  lime  silicates.  It  also  occurs  to  a  limited  extent  in  the 
gneisses.  It  is  most  frequently  accompanied  by  wollastonite,  diopside 
and  other  pyroxenes,  garnet,  epidote  and  titanite. 

Alteration  products  are  not  known  in  rock-making  vesuvianite.     It 


MELILITE.  159 

occasionally  encloses  the  substances  accompanying  it,  especially  calcite 
and  pyroxene ;  it  also  contains  fluid  inclusions ;  but  characteristic 
interpositions  of  any  kind  are  wanting,  and  the  mineral  is  most  fre- 
quently completely  homogeneous. 

Melilite. 
Literature. 

A.  STELZNER,  Ueber  Melilith  und  Melilithbasalte.    K  J.  B.  B.-B.  1882.  II.  369-387. 

cf.  N.  J.  B.  1882.  I.  229. 
F.  ZIRKEL,  Untersuchungen  iiber  die  mikroskopische  Zusammensetzung  und  Structur 

der  Basaltgesteine.     Bonn.  1870.  77  sqq. 

Bock-making  melilite  often  occurs  in  perfectly  crystallized  indi- 
viduals, and  then  has  the  form  of  quadratic,  octagonal,  or  rounded 
plates,  according  to  whether  the  prism  ccP  (HO)  alone,  or  with 
ooPoo  (100)  or  oo P3  (310),  is  combined  with  the  basal  plane.  More 
rarely  it  is  in  the  form  of  short  prisms.  When,  as  is  most  frequently 
the  case,  the  individuals  are  not  well  crystallized,  it  is  the  prism  zone 
which  is  least  developed,  producing  thin  plates  with  irregular  bounda- 
ries, the  basal  plane  also  being  uneven.  Hence  sections  of  melilite 
are  lath-shaped  parallel  to  <?,  and  quadratic,  octagonal,  or  rounded  at 
right  angles  to  c.  In  some  rocks  it  occurs  in  quite  irregular  grains, 
and  receives  its  outline  from  that  of  the  other  constituents,  which 
existed  in  the  rock  previous  to  its  crystallization. 

The  cleavage  parallel  oP  (001)  is  poorly  developed ;  and  sections 
inclined  to  the  basal  plane  exhibit  but  few  cleavage  lines,  often  only 
one,  to  which  the  extinction  is  parallel.  Irregular  cracks  sometimes 
traverse  the  longitudinal  sections. 

In  transmitted  light  melilite  is  either  colorless  or  yellowish  to 
brownish ;  and  even  the  apparently  colorless  sections,  when  compared 
with  actually  colorless  substances  (apatite,  nepheline,  leucite)  in  the 
same  thin  section  are  found  to  be  dull  yellowish,  with  a  tinge  of 
green  or  gray.  The  index  of  refraction  is  higher  than  for  the  asso- 
ciated colorless  silicates ;  the  double  refraction  is  very  weak.  In 
very  thin  sections  its  cross-sections  appear  almost  isotropic,  and  the 
double  refraction  can  only  be  noticed  by  observation  in  sensitive 
colors  (gypsum  plates,  etc.).  Even  in  thicker  sections  the  interfer- 
ence colors  do  not  exceed  gray-blue  of  the  1st  order.  But  the  degree 
of  its  double  refraction  appears  to  be  somewhat  different  in  different 
occurrences.  The  character  of  the  double  refraction  is  negative. 


160         PHYSIOGRAPHY  OF  THE  HOCK-MAKING  MINERALS. 

On  humboldtilite  from  Vesuvius— 

GOP  =  1.6312  ep  =  1.6262  (L.  Henniger) 

oona=  1.6339  ena=  1.6291 

A.  Michel  Levy*  determined  on  the  same  occurrence  &?  —  e  =  0.0058. 
In  convergent  light  basal  sections  yield  a  very  faint  cross,  and  the  optical 
character  has  to  be  determined  in  parallel  light.  Pleochroism  is  entirely 
absent  from  the  colorless  and  slightly  colored  melilite ;  for  the  decidedly 
yellow  variety  Stelzner  found  E  dark  yellow,  0  light  yellow. 

Melilite  possesses  a  peculiar  and  constant  micro-structure  in  rocks, 
which  may  be  used  to  advantage  with  its  essential  characteristics  as  a 
means  of  identification.  Longitudinal  sections  (\\c)  exhibit  either  fine 
lines  quite  like  cleavage  cracks,  which  traverse  the  section  parallel  to 
the  principal  axis  (PL  XIV.  Fig.  6),  or  there  starts  out  from  the  basal 
terminal  plane  curious  forms  shaped  like  pegs,  spears,  spatulse,  oars, 
or  pipes  (PI.  XV.  Fig.  6)  which  extend  to  a  greater  or  less  depth 
into  the  crystal,  and  sometimes  widen  but  to  spherical  or  funnel-shaped 
forms,  or  less  frequently  send  out  arms  parallel  to  oP  (001).  The  sub- 
stance filling  these  forms  appears  to  be  isotropic  glass.  Melilite  also  en- 
closes the  older  minerals  associated  with  it,  especially  augite  and  leucite  ; 
less  frequently  the  iron  ores,  oxides,  perofskite,  and  apatite.  The  arrange- 
ment of  the  inclusions  is  mostly  central  or  irregular,  more  rarely  zonal. 

Its  specific  gravity,  2.90-2.95,  allows  it  to  be  easily  separated  from 
the  rock  powder.  Melilite  is  a  lime-alumina  silicate,  12CaO,  2A12O3, 
9SiO2,  in  which  besides  lime  there  may  be  magnesia  and  alkalies,  and 
besides  alumina  there  may  be  sesquioxide  of  iron.  It  gelatinizes  very 
readily  in  hydrochloric  acid,  and  in  the  solution  sulphuric  acid  precipi- 
tates a  great  quantity  of  gypsum  needles.  It  is  frequently  found  de- 
composed in  nature;  almost  always  altering  to  a  fibrous  aggregate, 
possessing  strong  double  refraction,  and  probably  belonging  to  a  zeo- 
lite ;  the  fibres  generally  stand  perpendicular  to  the  basal  plane  of  the 
crystal,  and  penetrate  it  from  both  these  faces.  In  other  cases  the 
fibres  are  arranged  in  delicate  radial  groups.  By  incident  light  such 
more  or  less  altered  melilite  appears  chalk-white  or  ochre-colored  and 
earthy.  Tornebohmf  observed  an  alteration  of  melilite  to  garnet  in 
the  basalt  of  Alnd,  Sweden. 

Melilite  is  confined  to  the  younger  volcanic  rocks.  It  is  widely 
disseminated  in  the  leucite  and  nepheline  rocks.  It  also  forms  rocks 
in  which  it  takes  the  place  of  the  feldspars.  Besides  nepheline,  leucite, 
and  augite,  perofskite  is  a  constant  accompaniment  of  melilite. 

*  Bull.  Soc.  Min.  FT.  VII.  1884.  46. 

f  Geol.  Foren.  i.  Stockholm  Forhandl.     1882.  VI.   No.  76.  243. 


HEXAGONAL  MINERALS.  161 


MINEKALS  OF  THE  HEXAGONAL  SYSTEM. 

HEXAGONAL  minerals  are  anisotropic  and  uniaxlal,  like  tetragonal 
ones.  The  optic  axis  coincides  with  the  principal  crystallographic 
axis,  and  is  at  the  same  time  the  axis  of  greatest  or  least  elasticity. 
In  the  first  case  the  character  of  the  double  refraction  is  negative, 
and  GO  >  e;  in  the  second  case  it  is  positive,  and  GO  <  e.  The  two  rays 
are  differently  absorbed ;  and  hexagonal  minerals,  if  colored,  show  a 
more  or  less  distinct  pleochroisrn  in  all  sections  except  those  parallel 
to  the  basal  plane.  Sections  at  right  angles  to  c  have  hexagonal  or 
triangular  (in  tourmaline  nine-sided)  outlines,  or  directions  of  cleavage. 
Or  else  there  is  no  regular  outline,  and  the  cleavage  lies  parallel  to  the 
base  oP  (001).  Such  sections  act  like  isotropic  media  in  parallel  pol- 
arized light,  that  is,  they  remain  dark  between  crossed  nicols  during  a 
complete  rotation.  In  convergent  light  they  exhibit  a  dark  interference 
cross  with  or  without  colored  rings,  which  remains  unchanged  during  a 
rotation  of  the  section,  the  arms  of  the  cross  lying  parallel  to  the  prin- 
cipal sections  of  the  nicols.  Sections  which  are  inclined  or  parallel  to 
c  show  outlines  which  vary  with  the  position  of  the  section  and  the 
form  of  the  crystal ;  the  cleavage  appears  as  systems  of  cracks  running 
parallel  to  or  intersecting  one  another.  These  sections  are  doubly 
refracting  in  parallel  polarized  light ;  for  a  complete  rotation  of  the 
section  between  crossed  nicols  they  become  dark  and  light  four  times, 
and  the  position  of  darkness  is  always  reached  when  the  cleavage 
cracks  are  either  parallel  to  the  principal  sections  of  the  nicols  or  the 
latter  bisect  the  angles  made  by  intersecting  cleavage  lines.  In  con- 
vergent polarized  light  the  basal  interference  figure  sometimes  ap- 
pears at  one  side  of  the  field  of  view,  and  moves  in  the  margin  of 
,the  field  during  a  rotation  of  the  section  in  such  a  manner  that  the 
arms  of  the  cross  move  parallel  to  themselves.  Finally,  in  sections 
parallel  to  the  principal  axis  there  appear  hyperbolic  curves  lying  sym- 
metrical to  the  principal  axis. 

Optical  anomalies  are  recognized  in  convergent  light  by  the  inter- 
ference cross  in  basal  sections  opening  into  hyperbolas,  and  thus  present- 
ing the  interference  figure  of  a  biaxial Jbody  with  small  optic  angle  cut 
at  right  angles  to  the  first  bisectrix.  Different  parts  of  such  an  abnor- 
mal section  generally  show  different  sizes  of  apparent  axial  angle,  and 
different  positions  for  the  apparent  axial  plane.  In  parallel  polarized 
light  such  basal  sections  usually  exhibit  a  structure  resembling  twinning. 


162          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

From  the  foregoing  it  is  evident  that  tetragonal  and  hexagonal 
minerals  can  only  be  distinguished  by  their  form  and  cleavage,  but  not 
by  their  optical  characters. 

Graphite. 

The  graphite  which  occurs  in  rocks  generally  has  quite  irregular 
forms  :  it  constitutes  flakes  and  grains  of  very  variable  shape,  as  well 
as  disk-like  bodies ;  occasionally  it  shows  a  more  or  less  distinct  ap- 
proximation to  crystalline  outline,  and  then  possesses  hexagonal  to 
rounded  sections,  and  rectangular,  staff-shaped  longitudinal  sections. 
For  the  most  part  it  is  disseminated  in  minute  particles,  only  recog- 
nized as  such  with  high  magnifying  powers.  Graphite  is  opaque  ;  by 
incident  light  black  to  brownish  black  with  metallic  lustre.  It  is  not 
acted  on  by  acids  ;  is  consumed  only  with  difficulty  in  thin  sections  on 
platinum  foil,  even  after  the  iron  oxides  accompanying  it  have  been 
removed  by  acids. 

Graphite  =  C  is  widely  disseminated  as  a  constituent  or  a  pig- 
ment of  the  oldest  formations,  more  especially  of  the  phyllitic  kinds, 
where  it  is  evidently  the  residuum  of  organic  carbonaceous  sub- 
stances. It  also  occurs  under  similar  conditions  in  rocks  of  the  Ar- 
chaean, extending  down  deep  into  the  gneiss.  It  only  occurs  in  rocks 
of  more  recent  formations,  when  these  have  assumed  a  more  or  less 
crystalline  character  through  metamorphic  processes. 

Magnetic  Pyrites — Pyrrhotite. 

Pyrrhotite  never  forms  crystals  in  rocks,  but  always  in  irregular 
masses.  Its  outlines  are  therefore  irregular.  Cleavage  is  wanting.  It  is 
opaque  ;  by  incident  light  bronze  yellow,  with  distinct  metallic  lustre. 
It  is  distinguished  from  pyrite  by  its  color,  its  attraction  by  an  electro- 
magnet, as  well  as  by  its  solubility  in  hydrochloric  acid.  Chemical 
composition  =  FenSn+1.  It  occurs  occasionally  in  old  erruptive  rocks, 
is  especially  frequent  in  gabbro,  more  rarely  in  schists. 

Hematite. 

Hematite  occurs  in  three  different  forms  in  rocks — as  specular  iron, 
micaceous  hematite,  and  red  hematite. 

As  specular  iron  it  forms  rhombohedral  or  tabular  crystals,  with  a 
parting  parallel  to  the  faces  E  n  (lOll)  (86°  10')  which  is  often  dis- 
tinct, and  which  is  probably  to  be  referred  to  the  twinning  according  to 
the  rhombohedral  faces,  described  by  Bauer.*  This  twinning  is  probably 
a  mechanical  phenomenon,  as  Miiggef  is  inclined  to  think.  The  sections 

*  Z.  D.  G.  G.  1874.  XXVI.  186.  \  N.  J.  B.  1884.  I.  216. 


HEMATITE.  163 

parallel  to  the  base  are  then  triangular  or  hexagonal,  and  perpendicular 
to  the  base  are  mostly  lath-shaped. 

As  micaceous  hematite,  it  always  has  the  form  of  thin  plates  with 
hexagonal  outlines ;  the  sides  of  the  hexagons  are  often  of  very  un- 
equal length.  The  outlines  are  also  ragged,  or  quite  irregular.  The 
plates  are  often  aggregated  to  delicate  forms  of  growth  of  many  shapes, 
especially  when  they  occur  in  laminated  minerals  (mica),  when  their 
position  and  arrangement  are  dependent  on  the  crystallization  of  the 
matrix. 

As  red  hematite  it  is  massive,  and  forms  a  very  finely  divided  pig- 
ment in  rocks,  recognized  with  high  magnifying-powers  as  flakes  and 
grains,  or  as  loose  aggregates. 

Hematite  does  not  exhibit  any  cleavage  microscopically ;  but  the 
parting  parallel  to  the  fundamental  rhombohedron  gives  rise  to  lines, 
which  can  scarcely  be  distinguished  from  cleavage  cracks  microscopi- 
cally. 

As  specular  iron,  hematite  is  opaque,  with  metallic  lustre  by  incident 
light,  iron  black  to  grayish  black,  with  a  tinge  of  reddish,  which  is 
noticeable  in  a  strong  light. 

Micaceous  hematite  is  submetallic  in  lustre,  and  is  transparent,  with 
a  color  varying  with  the  thickness  of  the  plates  from  deep  red  through 
yellowish  red  to  yellowish  gray.  Pleochroism  is  not  noticeable.  Iso- 
lated plates  when  thin  enough  yield  a  uniaxial  interference  figure  in 
convergent  light. 

As  red  hematite  it  is  opaque,  reddish  by  incident  light,  without 
metallic  lustre. 

Specific  gravity  =  4.5-5.3.  Chemical  composition  =  Fe2O3.  Solu- 
ble in  hydrochloric  acid,  but  considerably  slower  than  magnetite. 
Not  attracted  to  a  simple  magnet  unless  attached  to  grains  of  mag- 
netite, which  character  may  be  used  as  a  means  of  separation  between 
the  two. 

Hematite  in  the  form  of  specular  iron  is  a  very  widely  disseminated 
constituent  in  the  acid  eruptive  rocks,  such  as  granite  and  syenite,  tra- 
chyte, rhyolite,  and  andesite ;  also  in  many  phonolites,  as  well  as  in  many 
crystalline  schists  of  the  Archaean.  In  the  eruptive  rocks  it  belongs 
to  the  oldest  individuals.  As  micaceous  hematite  it  occurs  in  the  same 
eruptive  rocks,  but  chiefly  as  inclusions  in  other  minerals  which  are 
colored  red  by  it :  thus  in  the  quartz,  feldspar,  and  mica  of  granites ; 
in  the  haiiynes  of  phonolites,  and  of  nepheline  or  leucite  rocks.  In 
the  crystalline  schists  it  occurs  both  independently  and  as  inclusions  in 
the  other  constituents.  The  red  color  of  the  phyllitic  schists  is  almost 


164          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

universally  due  to  tbe  presence  of  plates  of  micaceous  hematite.  More- 
over, this  form  of  hematite  is  the  most  widely  distributed  pigment  in 
the  mineral  world.  It  is  extremely  common  in  the  micas  of  the  peg- 
matitic  forms  of  granite  and  gneiss,  where  it  is  often  combined  with 
tourmaline  to  form  the  asterism  of  the  mica.  Such  occurrences  have 
been  described  by  G.  Hose,*  from  New  Providence  in  Pennsylvania, 
and  from  Grenville  in  Canada.  As  finely  divided,  loose,  red  hematite, 
it  is  present  in  the  acidic  porphyritic  rocks,  quartz  porphyries,  rhyo- 
lites,  quartz  porphyrites,  and  dacites.  It  colors  the  ground-mass  of 
these  rocks,  especially  when  they  assume  a  microfelsitic  development. 
Finally,  hematite  is  very  common,  partly  in  the  micaceous  form,  partly 
as  earthy  red  hematite,  in  pseudomorphs  after  pyrite  in  the  phyllitic 
schists,  as  well  as  in  pseudomorphs  after  olivine  and  bronzite  in  the 
basic  eruptive  rocks  (melaphyres,  basalts,  etc.) ;  lastly,  after  garnet  in 
eruptive  and  schistose  rocks. 

Ilmenite. 

The  development  of  ilmenite  in  rocks  is  exactly  similar  to  that  of 
hematite.  It  is  most  frequently  found  in  irregular  masses  without 
crystailographic  outline,  or  in  rhombohedral  crystals,  or  tabular  ones 
parallel  to  oR  (0001).  The  sections,  therefore,  are  either  irregular  or 
triangular  and  hexagonal  when  parallel  to  the  basal  plane,  or  often 
have  very  jagged  and  irregular  contours ;  perpendicular  to  the  base 
they  are  lath-shaped.  Frequently  ilmenite  plates  produce  incipient 
forms  of  growth  by  arranging  themselves  in  three  parallel  groups, 
which  cut  one  another  at  60°  in  cross-sections.  Another  kind  of  occur- 
rence strongly  resembles  the  micaceous  variety  of  hematite,  being  in 
very  thin  plates.  It  may  be  designated  as  micaceous  titanic  iron. 
Finally,  ilmenite  is  found  in  an  ochre-like  form,  and  appears  as  minute 
particles  and  aggregates,  and  serves  as  a  pigment  to  minerals  enclosing 
it  in  a  finely  divided  state.  "When  perfectly  fresh,  ilmenite  exhibits  no 
microscopic  cleavage  cracks  ;  as  soon,  however,  as  chemical  alteration 
sets  in,  a  system  of  stripes  and  lines  appears  in  cross-sections,  which 
may  follow  the  cleavage  parallel  to  R  it  (lOll).  But  since  there  are 
striations  noticeable  by  incident  light  on  the  natural  basal  plane  of 
even  the  freshest  individuals,  which  appear  parallel  to  the  intersection 
oiR-Tt  (1011),  and  arise  from  twinned  lamellse,  so  the  apparent  cleavage 
noticeable  in  partly  decomposed  sections  is  to  be  explained  as  a  shelly 
structure  parallel  to  It,  which  becomes  more  noticeable  through  decom- 

*  8.  B.  A.  1869,  19  Apr.,  p.  352  sq. 


ILMEN1TE.  165 

position.  Moreover,  a  shelly  structure  parallel  to  oR  (0001)  is  some- 
times observed  microscopically  in  cross-sections. 

Ilmenite  is  opaque,  with  metallic  lustre;  by  incident  light  iron- 
black,  with  a  tinge  of  brownish.  Micaceous  titanic  iron,  as  K.  Hof- 
mann  first  showed,  becomes  transparent  with  a  clove-brown  color,  and 
is  quite  strongly  doubly  refracting,  mostly  with  a  sub-metallic  lustre. 
The  ochre-like,  finely  divided  ilmenite  loses  the  metallic  habit,  and  by 
incident  light  is  brownish  black  and  dark  brown. 

Specific  gravity  =  4.3-4.9.  Chemical  composition  =  FeTiO3, 
when  pure ;  but  there  appears  to  be  quite  a  complete  series  between 
specular  iron  FeFeO3  and  FeTiO3,  in  which  there  also  occur  mem- 
bers carrying  MgTiO3.  Hot  hydrochloric  acid  attacks  ilmenite 
somewhat  more  slowly  than  it  does  specular  iron.  The  solution  when 
heated  with  tin-foil  becomes  violet.  Hot  concentrated  sulphuric  acid 
yields  a  blue  solution.  Pure  ilmenite,  like  hematite,  is  somewhat  in- 
different toward  the  magnet ;  a  distinct  attraction  toward  the  magnet 
indicates  an  admixture  of  magnetite.  It  is  with  difficulty  distinguished 
from  titaniferous  1-nagnetite,  when  neither  the  crystal  form  nor  shelly 
structure  nor  cleavage  permits  the  determination  of  its  system  of  crys- 
tallization. 

Ilmenite,  as  it  occurs  in  rocks,  is  very  frequently  more  or  less 
completely  altered  into  other  substances.  In  most  cases  this  alteration 
commences  with  the  formation  of  a  strongly  refracting  substance,  only 
slightly  transparent,  which  when  sufficiently  thin  is  strongly  doubly 
refracting.  Its  color  is  white,  yellow,  or  brown  by  incident  light ;  and 
its  structure  is  sometimes  granular,  sometimes  distinctly  radiating,  the 
fibres  standing  perpendicular  to  the  ilmenite.  This  alteration  product, 
which  may  also  arise  from  titaniferous  magnetite  and  from  rutile,  was 
called  leucoxen  by  Giimbel,  who,  however,  considered  its  substance  as 
of  primary  nature.  Its  chemical  composition  is  not  the  same  in  all 
cases  where  it  has  been  investigated,  and  it  has  been  considered  the 
equivalent  of  a  variety  of  minerals  (titariite,  anatase,  and  siderite)  by 
different  observers.  Since  leucoxen  grows  at  the  expense  of  the  ilmen- 
ite during  the  process  of  alteration,  its  outlines  possess  similar  geomet- 
rical forms  to  those  of  ilmenite  (PL  XYI.  Fig.  1).  When  there  is  a 
shelly  structure  parallel  to  R  7r(1011)  or  oR  (0001),  the  pseudo- 
morph  follows  these  shells  (PI.  XYI.  Fig.  2),  working  from  their  faces 
inward  until  the  whole  is  transformed.  A.  Cathrein  has  shown  that 
the  brown  and  yellow  color  of  many  leucoxens  is  due  to  the  mechani- 
cal mixture  of  rutile  in  the  form  of  sagenite,  which  already  existed 
intergrbwn  with  the  ilmenite. 


166         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

This  alteration  of  ilmenite  to  lencoxen  takes  place  both  in  eruptive 
rocks  and  in  schistose  ones.  But  so  far  as  experience  goes,  the  altera- 
tion of  ilmenite  into  carbonates  rich  in  iron,  in  which  either  the  previ- 
ously existing  rutile  remains  as  such,  or  the  titanic  acid  contained  in 
the  ilmenite  is  converted  into  rutile,  is  confined  to  schistose  rocks  of 
the  phyllite  series  and  to  phyllitic  schists  of  more  recent  formations. 
On  the  other  hand,  according  to  von  Lasaulx  it  is  very  probable  that 
ilmenite  may  at  times  be  derived  from  rutile. 

The  distribution  of  titaniferous  iron  in  the  form  of  ilmenite  is  very 
great.  It  accompanies  or  replaces  specular  iron  in  granites,  syenites, 
etc.;  it  belongs  to  the  essential  ingredients  in  diorites,  but  especially 
in  diabase,  gabbro,  and  related  rocks,  as  well  as  in  their  mesozoic  and 
more  recent  equivalents,  augite  porphyrites,  melaphyres,  basalts,  etc. 
In  these  rocks  ilmenite,  together  with  magnetite,  which  often  accom- 
panies it,  belongs  to  the  oldest  secretions  from  the  magma ;  its  forma- 
tion precedes  that  of  olivine  and  pyroxene,  and  seldom  appears  in  the 
later  stages  of  the  development  of  the  rocks.  Ilmenite  frequently 
forms  a  constituent  of  gneiss  and  mica-schist,  of  th%labradorite  rocks 
of  Norway  and  of  Canada,  and  of  amphibolite  from  many  localities. 
In  the  form  of  micaceous  titanic  iron  it  occurs  in  the  basalts  of  south- 
ern Bakony,  Hungary  ;  in  the  angite-  porphyrites  and  melaphyres  of 
the  Saar  !N"ahe  region,  Lower  Rhine ;  as  well  as  in  the  nepheline  basalts 
and  pyroxenites  of  Kaisersthul.  Ochreous  titanic  iron  probably  forms 
the  dust-like  pigments  which  often  give  to  the  plagioclases  of  gabbros 
and  ophites  their  peculiar  brown  color;  the  globulites  of  the  basic 
rock  glasses  (augite  porphyrite  and  basalt)  are  very  likely  titanic  iron. 

Corundum. 

The  forms  of  rock-making  corundum  vary  greatly.  At  times  it 
crystallizes  in  long  prismatic  shapes,  at  times  in  sharp  pyramids  or  in 
thinly  tabular  forms,  and  several  such  types  may  occur  together 
(PI.  XYI.  Fig.  3).  Cross-sections  parallel  to  the  base  are  hexagonal 
or  rounded,  those  parallel  to  0,  lath-shaped ;  but  the  longitudinal 
direction  of  the  section  corresponds  in  some  cases  to  that  of  the  prin- 
cipal axis,  in  others  to  that  at  right  angles  to  it.  It  is  often  only  possi- 
ble to  distinguish  between  these  types  by  means  of  an  optical  deter- 
mination. Furthermore,  corundum  forms  irregular  grains  and  masses. 
Cleavage  is  only  observed  in  the  larger  individuals  of  corundum  ;  it  is 
parallel  to  R  n  (1011).  The  concentric  structure  also,  which  is  deter- 
mined by  the  twinning  parallel  to  R  n  (1011),  is  seldom  observed  in 
microscopic  individuals. 

Corundum  is  generally  almost  colorless  or  transparent  blue,  seldom 


BRUCITE.  167 

brown  or  red.  The  color  is  not  disseminated  uniformly,  but  is  usually 
in  quite  irregular  patches  and  streaks,  or  in  concentric  zones.  Corundum 
has  a  high  index  of  refraction,  but  a  low  double  refraction  ;  Osann 
found  for  corundum  from  Ceylon  oona  =  1.7690,  ena -=  1.7598.  The 
character  of  the  double  refraction  is  negative.  The  relief  and  the 
dark  border  of  total  reflection  as  well  as  the  rough  surface  are  strongly 
marked ;  the  interference  colors  are  low,  and  in  good  thin  sections  do 
not  exceed  red  of  the  first  order.  Basal  sections  show  the  cross,  but 
without  rings  in  thin  sections  of  the  normal  thickness,  and  the  arms  of 
the  cross  are  somewhat  indistinct.  Optical  anomalies  common  to  the 
larger  individuals  are  mostly  wanting  in  microscopic  individuals. 
Pleochroism  is  only  strong  when  the  coloring  is  quite  deep ;  for  the  blue 
corundums  (sapphire  and  emery),  0  is  blue,  E  is  sea-green  to  colorless. 

H.  =  9.  Sp.  gr.  =  3.9-4.0.  Chemical  composition  =  A12O8. 
Corundum  is  not  attacked  by  acids,  and  is  not  dissolved  by  fused  soda; 
it  is  therefore  easily  separated  out  of  the  rocks. 

Rock-making  corundum  possesses  no  constant  microstructure;  most 
frequently  it  encloses  gas  and  fluid  inclusions,  which  latter  are  often 
found  to  be  liquid  carbon  dioxide.  It  has  often  grown  in  contact  with 
ilmenite,  and  also  encloses  it.  Rutile  crystals  and  sagenite  webs  also 
occur  frequently  in  the  larger  crystals,  seldom  in  the  microscopic  ones. 

Corundum  never  occurs  as  an  essential  constituent  of  rocks,  with 
the  exception  of  emery,  which,  together  with  iron  oxides,  forms  inde- 
pendent bodies  in  the  crystalline  schists.  It  appears  only  as  an 
accessory  constituent  in  granites,  gneisses,  granular  limestones  and 
dolomites,  and  is  constantly  accompanied  by  spinel,  rutile,  and  silli- 
manite.  Corundum  occurs  with  magnetite  and  pleonaste  as  a  contact 
mineral  in  connection  with  the  norites  of  the  Cortlandt  series  at  Stony 
Point,  on  the  Hudson,  N".  Y..*  and  in  the  dunite  and  serpentine  of 
Pennsylvania  and  North  Carolina. 

Brucite. 

Brucite  in  rocks  forms  irregular  as  well  as  hexagonal  plates,  seldom 
fibrous  aggregates.  When  in  the  form  of  plates,  the  basal  sections 
are  six-sided,  rounded  or  irregular,  and  also  ragged  ;  sections  parallel  to 
c  give  narrow  lath-shaped  figures. 

The  perfect  cleavage  parallel  to  oR  (0001)  is  very  clearly  expressed 
microscopically  by  fine  cracks,  which  run  parallel  to  one  another  and 
to  the  longitudinal  direction  of  the  lath-shaped  sections.  Basal  sec-, 
tions  exhibit  no  cleavage,  but  sometimes  show  cracks  and  curves  which 
do  not  appear  to  possess  any  regular  course. 

*  G.  H.  Williams,  Am.  Journ.  Sci.  XXXIII.  Feb.  1887.  198. 


168          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

Brucite  is  transparent  and  colorless,  its  index  of  refraction  about 
the  same  as  that  of  Canada  balsam  ;  the  double  refraction  is  strong  and 
positive.  Bauer  determined  cop  =  1.560,  ep  =  1.581.  Basal  sections 
exhibit  a  very  distinct  interference  cross  in  convergent  light ;  upon 
rotation  this  cross  very  often  opens  into  hyperbolas,  the  position  of 
whose  poles  varies  for  different  parts  of  the  section.  The  figure 
corresponds  to  that  of  a  biaxial  medium  with  small  optic  angle  cut  at 
right  angles  to  the  acute  bisectrix.  The  fact  that  not  only  the  appar- 
ent axial  angle  but  often  the  position  of  the  axial  plane  varies  in  the 
same  section,  indicates  that  the  source  of  the  phenomena  is  due  to 
strains.  In  parallel  polarized  light  such  basal  sections  are  not  homoge- 
neous and  isotropic,  but  exhibit  streaked  or  striped  doubly  refracting 
areas  with  very  dull  polarization  colors.  Longitudinal  sections  in 
parallel  polarized  light,  when  the  cleavage  cracks  are  inclined  45°  to 
the  principal  planes  of  the  nicols,  are  brightly  colored,  and  may  be 
easily  mistaken  for  muscovite  and  talc,  which,  however,  are  still  more 
strongly  doubly  refracting.  They  may  be  distinguished  by  the  fact 
that  for  brucite  the  elasticity  of  the  ether  parallel  to  the  cleavage  is 
greater  than  that  perpendicular  to  it,  while  for  muscovite  and  talc  the 
reverse  is  true. 

Sp.  gr.  =  2.3-2.4.  Chemical  composition  =  MgO,  HaO.  It  is 
soluble  in  acids ;  upon  being  heated  to  redness  on  platinum  foil  it  is 
sometimes  colored  pink  through  the  oxidation  of  a  trace  of  FeO  to 
Fe2O3.  If  a  thin  section  containing  brucite  be  moistened  with  a  solu- 
tion of  silver  nitrate  after  it  has  been  slightly  heated  to  redness  on 
platinum  foil,  the  brucite  quickly  turns  brown  through  the  deposition 
of  oxide  of  silver. 

Brucite  is  disseminated  in  small  quantities  in  phyllites  containing 
magnesite  or  dolomite,  as  well  as  in  similar  crystalline  schists,  in 
actinolite  schists,  and  also  in  serpentines  and  many  decomposed  dia- 
bases. In  these  cases  it  has  evidently  been  derived  from  the  carbonates 
and  silicates  of  magnesia.  It  also  occurs  as  a  contact  mineral  in  some 
granular  limestones. 

Quartz. 

When  rock-making  quartz  possesses  crystal  form  it  appears  in  di- 
hexahedrons  ±  It  (1011),  on  which  the  prism  ooT?  (1010)  is  but  rarely 
developed,  and  then  only  to  a  slight  extent.  Hence  sections  parallel 
to  the  base  give  regular  hexagons ;  those  in  the  prism  zone  rhombs, 
with  angles  of  76°  26'  and  103°  34' ;  inclined  sections  have  triangular 
or  trapezoidal  outlines.  On  account  of  the  very  general  rounding  of 


QUARTZ.  169 

the  edges  and  corners,  the  sections  often  appear  round ;  in  conse- 
quence of  mechanical  deformation  of  crystals  which  were  originally 
regularly  bounded,  the  outlines  of  the  fragments  are  irregular  and 
sharply  angular ;  curved  and  looped  contours  are  due  to  the  corrosion 
of  completed  crystals  (PL  Y.  Fig.  1).  Under  certain  conditions  the 
quartz  of  later  generation  in  many  porphyritic  (granophyric)  eruptive 
rocks  appears  to  crystallize  in  peculiar  forms  of  growth,  which  are 
analogous  to  the  quartz  of  graphic  granite ;  the  habit  then  is  appar- 
ently prismatic  or  trapezohedral.  The  individuals  are  intergrown  in 
the  most  intimate  manner  with  feldspar  (orthoclase,  albite,  or  oligo- 
clase),  and  within  one  and  the  same  feldspar  crystal  they  lie  exactly 
parallel  to  one  another  (PI.  VIII.  Fig.  3). 

In  by  far  the  greatest  number  of  rocks  the  quartz  is  massive  with- 
out crystal  form,  and  its  outline  is  consequently  of  no  determinative 
value.  This  sort  of  quartz  sometimes  forms  single  individuals,  some- 
times aggregates ;  the  latter  are  almost  always  granular.  Only  in  the 
spherulites  of  porphyritic  rocks  or  where  the  quartz  is  pseudomor- 
phous  after  a  fibrous  mineral  is  it  fibrous.  In  the  first  case  it 
seems  to  be  necessary  to  refer  it  to  chalcedony,  which  hardly  occurs 
in  any  other  form  than  fibrous. 

In  the  rock-making  quartzes  there  is  no  trace  of  the  interpenetra- 
tion  twinning  so  characteristic  of  attached  crystals  of  quartz.  This 
may  be  due  to  the  fact  that  the  ordinary  twinning  in  which  the  systems 
of  axes  are  parallel  could  not  be  detected  optically  on  account  of  the 
extreme  thinness  of  the  rock  sections. 

The  imperfect  cleavage  parallel  to  the  rhombohedron  is  very  rarely 
met  with  in  microscopic  quartz,  and  cleavage  cracks  are  almost  entirely 
absent.  A  rhombohedral  shelly  structure  is  occasionally  recognized 
by  the  mode  of  arrangement  of  inclusions,  especially  fluid  inclusions. 
The  absence  of  cleavage  is  one  of  the  most  characteristic  negative  cri- 
teria of  quartz  under  the  microscope. 

In  thin  sections  rock-making  quartz  is  transparent  and  colorless, 
even  when  it  appears  colored  by  incident  light.  The  milky  clouding 
in  incident  light  is  mostly  due  to  inclusions  of  fluids  and  gases.  The 
source  of  the  blue  color  of  many  granitic  quartzes  is  not  yet  definitely 
known  ;  the  red  color  of  the  quartzes  of  silicious  rocks,  seen  by  inci- 
dent light,  arises  from  minute  plates  of  hematite  and  ilmenite ;  the 
green  color  in  those  of  many  hornblendic  schists  is  due  to  needles  of 
amphibole;  the  jet-black  and  blue-black  color  of  the  quartz  in  many 
porphyroids  and  phyllitic  rocks  is  caused  by  carbonaceous  substances 
(graphite,  coal),  rarely  by  magnetite.  The  index  of  refraction  of 


170          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

quartz  is  almost  the  same  as  that  of  Canada  balsam,  consequently  the 
surface  of  the  section  is  perfectly  plain  and  without  relief.  The 
double  refraction  is  weak  and  its  character  positive.  Rudberg  found 
oona  —  1.54418,  ena  =  1.55328.  The  interference  colors  of  quartz  in 
thin  sections  scarcely  ever  exceed  those  of  the  1st  order;  in  good  thin 
sections  it  shows  the  bluish  or  yellowish  tints  of  the  1st  order.  Basal 
sections  give  the  interference  cross  without  rings,  and  permit  the  posi- 
tive character  to  be  easily  determined  with  the  quarter  undulation 
mica  plate.  This  is  the  most  important  optical  means  of  determina- 
tion for  quartz,  taken  in  connection  with  its  weak  double  refraction 
and  transparency.  Circular  polarization  does  not  appear  in  thin  sec- 
tions because  of  their  thinness. 

Sp.  gr.  =  2.65.  Chemical  composition  =  SiO2.  Not  attacked  by 
ordinary  acids ;  is  dissolved  by  hydrofluoric  acid,  which  acts  slowly 
on  thin  sections.  The  resistance  which  quartz  offers  to  all  the  re- 
agents occurring  in  nature  accounts  for  the  fact  that  it  never  appears 
weathered  in  thin  sections,  but  is  always  completely  fresh. 

The  following  varieties  of  rock-making  quartz  may  be  conveni- 
ently distinguished : 

Granitic  quartz.  Massive,  and  with  its  outline  determined  by  those 
of  the  minerals  associated  with  it.  It  forms  the  youngest  primary 
constituent  of  the  acid  granular  eruptive  rocks,  either  as  an  essential  or 
an  accessory  ingredient;  thus  in  granites,  syenites,  diorites,  and  certain 
diabases.  Granitic  quartz  is  highly  characterized  by  an  abundance  of 
fluid  inclusions.  These  are  mostly  in  irregular  swarms  and  streaks, 
but  are  sometimes  arranged  in  planes  parallel  to  the  rhombohedral 
faces.  The  fluidal  cavities  are  sometimes  completely  filled  with  fluid 
(water,  more  rarely  liquid  carbon  dioxide,  or  both),  or  there  may  be  a 
gas  bubble  present.  The  relative  sizes  of  the  bubble  and  fluid  vary 
within  the  widest  limits.  Besides  the  fluid  inclusions  occur  gas  inclu- 
sions, whose  contents  are  probably  water  vapor.  In  the  fluid  inclusions 
there  are  sometimes  crystalline  secretions,  usually  of  cubical  form,  which 
may  in  some  cases  be  sodium  chloride.  Besides  these  inclusions,  granitic 
quartzes  often  contain  extremely  fine  opaque  microlites,  which  Hawes 
referred  to  rutile.* 

Massive  granitic  quartz  often  bears  the  traces  of  mechanical  de- 
formation in  the  peripheral  shattering  of  the  larger  grains,  as  well  as 
in  the  wavy  extinction  due  to  a  continuous  change  in  the  direction  of 

*  Mineralogy  and  Lithology  of  New  Hampshire.     Concord.  1878.  45, 


QUARTZ.  171 

the  principal  axis  in  one  and  the  same  grain.  This  deformation  is 
undoubtedly  the  result  of  mountain-making  forces. 

Closely  related  to  granitic  quartz  is  the  quartz  of  the  crystalline 
and  phyllitic  schists  ;  this  also  is  massive,  and  destitute  of  outward 
crystalline  boundary.  But  it  does  not  receive  its  form  from  the  mine- 
rals associated  with  it  to  the  same  extent  as  the  granitic  quartz  does. 
They  rather  mutually  penetrate  one  another,  especially  in  the  case  of 
feldspar.  The  forms  are  rounded  to  lenticular,  and  range  from  micro- 
scopic grains  to  those  of  very  considerable  dimensions.  The  mutual 
intergrowth  with  feldspar  (occasionally  also  with  garnet,  hornblende 
and  other  minerals)  is  similar  to  the  granophyric  structure  of  certain 
eruptive  rocks.  The  inclusions  correspond  in  all  points  to  those  in 
granitic  quartz,  and  the  phenomena  of  mechanical  deformation  are  still 
more  widespread. 

Porphyritic  quartz  should  properly  exhibit  a  well-developed  crys- 
tal form,  which,  however,  may  be  more  or  less  completely  lost  through 
chemical  corrosion  or  mechanical  deformation.  Its  forms  never  show 
a  dependence  on  those  of  the  associated  minerals,  and  it  is  evident  that 
this  quartz  was  formed  at  a  time  when  more  or  less  of  the  rock  existed 
in  the  condition  of  magma.  Porphyritic  quartz  is  an  essential  con- 
stituent of  quartz  porphyry,  quartz  porphyrite  rhyolite  (liparite),  and 
dacite  (quartz  andesite).  Gas  and  fluid  inclusions  are  found  here  as  in 
granitic  quartz,  but  generally  not  in  such  quantities ;  they  are  accom- 
panied by  the  very  characteristic  glass  inclusions  of  round  and  dihexa- 
hedral  form  (PI.  YII.  Figs.  2  and  3).  Phenomena  of  mechanical 
deformation  are  quite  rare,  except  those  in  the  closely  related 
quartzes  of  the  porphyroides  which  are  without  glass  inclusions ;  the 
greater  part  of  them  are  from  strains  produced  by  glass  inclusions. 
Fracturing  caused  by  the  fluid  movement  in  the  plastic  rock  mass 
is  common.  The  chemical  corrosion  of  porphyritic,  quartzes  (PLY. 
Fig.  1)  is  highly  characteristic  and  distinguishes  them  from  those  of 
granites  and  schists.  Not  infrequently  in  the  porphyritic  rocks  the 
quartz  assumes  spherical  forms,  which  sometimes  consist  of  a  single 
individual,  sometimes  of  two  or  three,  seldom  of  more,  which  then 
appear  as  spherical  sectors.  This  has  been  called  quartz  globulaire  by 
Michel-Levy.  The  substance  of  these  forms  is  often  mixed  with  more 
or  less  microfelsite. 

The  clastic  quartz  of  sandstones,  graywackes,  and  related  rocks  is 
usually  without  crystal  form,  being  angular  or  rounded  ;  the  shape  of 
the  grains  is  not  determined  by  its  aggregation  with  other  minerals, 
but  by  the  mechanical  processes  which  took  part  in  its  deposition. 


172          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

The  microstructure  of  the  separate  grains  is  that  of  the  particuk. 
variety  of  quartz  to  which  it  originally  belonged.  Naturally  the 
microstructure  of  the  granitic  and  gneissic  quartzes  predominates.  By 
the  secondary  deposition  of  silica  in  crystallographic  orientation 
around  the  clastic  grains  the  latter  may  assume  the  crystal  form*  (so- 
called  crystallized  sandstones,  etc.).  Such  regenerated  quartzes  are 
not  uncommon  in  the  clay  slates.  The  so-called  quartz  of  the  siliceous 
.slates  and  of  related  rocks  requires  more  exact  investigation,  and 
probably  does  not  belong  to  quartz,  but  to  chalcedony. 

Gangue  quartz  forms  irregular  masses  in  granular  aggregates, 
whose  microstructure  closely  resembles  that  of  the  gneissic  quartz  on 
account  of  the  abundance  of  fluid  and  gas  inclusions.  To  this  variety 
belong  the  secondary  quartz  in  rocks  of  all  classes,  which  arises  from 
the  decomposition  and  weathering  of  the  silicates.  It  often  occurs  in 
pseudomorphs  after  these  and  other  minerals  (feldspar,  mica,  horn- 
blende, pyroxene,  etc.).  Occasionally  the  quartz  in  these  pseudo- 
morphs is  fibrous,  but  only  when  the  original  mineral  was  fibrous,  as 
fibrous  calc  spar,  asbestus,  chrysotile,  crocidolite,  etc. 

Chalcedony. 

Chalcedony  forms  concretionary  crystalline  masses,  mostly  with  a 
radially  fibrous  structure  and  shelly  parting,  rarely  with  a  parallel 
fibrous  structure.  The  fibres  always  appear  to  stand  perpendicular  to 
the  surface  of  the  shells  or  layers ;  they  have  very  variable  dimensions 
transversely,  but  are  always  extremely  thin.  Within  the  solid  rock 
chalcedony  is  generally  in  the  form  of  spherulites  (PL  IX.  Fig.  1), 
while  in  cracks  and  cavities  it  occurs  as  a  coating  or  in  stalactites.  The 
crystal  system  of  chalcedony  has  not  yet  been  definitely  determined  ; 
however,  it  appears  to  be  optically  uniaxial.  The  index  of  refraction  is 
smaller  than  for  quartz,  np  =  1.53T ;  the  double  refraction  is  some- 
what stronger.  The  character  of  the  double  refraction  Js  negative, 
which  may  be  detected  by  examining  the  spherulites  with  a  quartz 
wedge  or  a  gypsum  plate.  This  characteristic  permits  of  its  being 
readily  distinguished  from  quartz.  Tangential  sections  exhibit  a  fine- 
grained aggregate  polarization,  and  the  small  dimensions  of  the  fibres 
prevent  their  possible  uniaxial  nature  from  being  determined.  Central 
sections  through  the  concretions  give  the  spherulitic  interference  cross. 

Sp.  gr.  =  2.59-2.64,  somewhat  lower  than  for  quartz.  Chemical 
composition  —  SiO2,  with  the  same  chemical  behavior  as  for  quartz. 

*  R.  D.  Irving,  Am.  Journ.  Sci.   June,  1883,  and  Irving  and  Van  Hise,  Bulletin, 
:No.  8.,  U.  S.  Geol.  Survey.    1884. 


TRIDYMITE,  173 

Chalcedony  appears  as  an  original  constituent  of  the  ground  mass 
of  very  silicious  porphyritic  eruptive  rocks  which  have  a  microfelsitic 
development;  thus  in  many  quartz  porphyries,  rhyolites  (liparites), 
quartz  porphyrites,  and  dacites.  Moreover,  the  spherulites  of  silica 
in  quartz  slates  and  related  rocks  appear  to  belong  to  chalcedony. 
As  a  decomposition  product  it  occurs  in  all  kinds  of  silicate  rocks. 

Tridymite. 
Literature. 

A.  VON  LASAULX,  Ueber  das  optische  Yerhalten  und  die  Krystallform  des  Tridymits, 

Z.  X.  1878.  II.  253-274. 

A.  MERIAN,  Beobachtungen  am  Tridymit.  N.  J.  B.,  1884.  I.  193-195. 
M.  SCHUSTER,  Optisches  Verhalten  des  Tridymit  aus  den  Euganaen.     T.  M.  P.  M, 

1878.  I.  71-77. 
F.  ZIRKEL,  Ueber  den  mikroskopischen  Tridymit.  Pogg.  Ann.  1870    CXL.  492. 

Tridymite  forms  tabular  crystals,  sometimes  with  rounded  outlines 
which  are  bounded  by  the  planes  oP  (0001)  and  &P  (1010).  In  the 
attached  crystals  in  cavities  there  occur  in  addition  to  the  derived 
pyramids  the  prism  of  the  2d  order,  and  a  dihexagonal  prism  often 
developed  hemihedrally.  Moreover,  the  attached  crystals  are  almost 
always  juxtaposition  or  penetration  trillings  along  the  faces  \P  (1016) 
and  \P  (3034),  while  in  the  rock-making  crystals  this  twinning  does 
not  seem  to  occur.  The  dimensions  of  the  rock-making  tridymites  are 
always  microscopic;  consequently  they  almost  never  appear  as  sections, 
but  as  complete  bodies.  The  microscopic  individuals  occur  almost 
without  exception  in  tile-like  aggregates,  in  which  the  faces  oP  overlap 
one  another  (PL  XYI.  Fig.  4).  Through  the  suppression  of  one  pair 
of  prism  faces  the  outline  is  often  rhombic. 

Cleavage  is  not  known  in  rock-making  tridymite,  though  there  is 
sometimes  a  parting  parallel  to  oP  (0001)  due  to  the  growing  together 
of  several  plates  along  this  face. 

Tridymite  as  a  rock  constituent  is  free  from  inclusions,  with  the 
exception  of  gas  interpositions ;  it  is  transparent  and  pellucid,  with  a 
weaker  refraction  than  Canada  balsam  ;  and  with  moderate  double  re- 
fraction whose  character  is  positive.  The  plates  which  have  grown  in 
the  rock  behave  isotropic  when  they  lie  on  their  basal  plane,  and 
weakly  doubly  refracting  in  other  positions.  The  larger  attached 
crystals  often  exhibit  in  basal  planes  a  very  complicated  division  into 
areas  which  are  doubly  refracting,  and  which  show  the  locus  of  optic 
axes  or  a  bisectrix  in  convergent  light,  which,  would  indicate  that  tliese 


174          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

areas  belong  to  the  triclinic  system.  Such  tridymite  plates  become 
isotropic  upon  being  heated,  but  resume  their  doubly  refracting  char- 
acter when  allowed  to  cool.  From  this  it  is  inferred  that  the  tridymite 
plates  crystallized  originally  in  the  hexagonal  system,  but  that  under  the 
physical  conditions  existing  at  the  surface  of  the  earth  they  become  sub- 
jected to  strains  in  an  attempt  to  assume  another  molecular  arrangement. 

Sp.  gr.  =  2.28-2.33.  Chemical  composition  =  SiO2 ;  chemical  be- 
havior the  same  as  for  quartz,  except  that  tridymite  is  soluble  in  boil- 
ing caustic  soda. 

Tridymite  is  chiefly  a  volcanic  mineral ;  it  is  a  frequent  constituent 
of  rhyolite  (liparite),  trachyte,  and  andesite.  It  is  particularly  abun- 
dant in  the  lithophysae  of  obsidian  and  rhyolite  in  the  Yellowstone 
National  Park.  'It  has  also  been  found  by  G.  Eose  in  the  opals  of 
Kosemiitz,  Silesia;  Kashan,  Persia;  and  Zimapan,  Mexico;  and  in 
the  cacholong  of  Iceland.  In  an  augite  andesite  from  Grad-Jakan  in 
Java  it  occurs  probably  through  the  decomposition  of  feldspar.  Tri- 
dymite has  also  been  found  in  meteorites. 

Calotte. 

As  a  rock-making  constituent  calcite  never  occurs  in  crystals :  it  is 
either  in  irregularly  bounded  grains  and  plates  or  in  aggregates,  or  it 
occurs  in  parallel  or  radiating  fibrous  aggregations,  or  else  it  presents 
that  peculiar  concretionary,  crystalline  form  called  oolitic.  Hence  its 
crystal  form  is  of  no  importance  for  its  microscopical  determination, 
Still  the  grains  and  plates  of  calcite  found  in  rocks  are  mostly  charac- 
terized by  the  twinning  parallel  to  —  \  JR  it  (0112),  by  which  each 
grain  is  converted  into  a  polysynthetic  individual  (PL  XVI.  Fig.  5). 
This  polysynthetic  twinning  is  of  very  common  occurrence  in  crystal- 
line limestone,  and  may  very  likely  have  been  produced  by  pressure  ; 
it  can  also  be  produced  during  the  process  of  grinding  when  the  sec- 
tion is  sufficiently  thin. 

The  calcite  cleavage  parallel  to  R  n  (1011)  shows  itself  in  thin 
section  by  numerous  sharp  cracks  (PI.  X.  Fig.  1),  whose  inclination  to 
one  another  changes  with  the  position  of  the  section.  This  cleavage  is 
one  of  the  most  important  means  of  distinguishing  calcite  from  other 
minerals,  with  the  excption  of  dolomite,  magnesite,  etc.  Along  the 
cleavage  cracks,  which  do  not  cut  the  calcite  sections  perpendicularly, 
there  often  appear  Newton  colors  produced  by  the  interference  of  the 
light  reflected  back  and  forth  from  the  sides  of  the  cracks. 

jOalcite  when  pure  is  colorless,  but  appears  dark  gray,  bluish,  almost 
opaque,  brownish  or  yellowish  in  transmitted  light  in  consequence  of 


CALCITE.  175 


organic  pigments.  The  mean  index  of  refraction  is  not  high,  conse- 
quently the  surface  of  the  sections  is  plain,  and  the  relief  small.  On 
the  other  hand,  the  double  refraction  is  very  strong,  with  negative  char- 
acter ;  cona  =  1.6585,  eno  =  1.4864.  Hence  the  interference  colors  be- 
tween crossed  nicols  are  high,  even  for  very  thin  sections ;  during  a 
rotation  between  crossed  nicols  darkness  alternates  with  clear  white  or 
pale  green  and  bright  pale  green,  and  the  more  precise  colors  of  the 
lower  orders  are  wanting.  Basal  sections  in  convergent  polarized  light 
give  an  interference  cross  with  several  colored  rings,  even  for  very 
thin  sections.  The  same  thing  is  obtained  from  radial  aggregates 
which  sometimes  occur  as  secondary  minerals  in  eruptive  rocks,  when 
the  section  is  tangential  and  the  objective  of  the  microscope  is  not 
focused  on  the  surface  of  the  section,  but  on  a  point  in  which  the 
rays  of  like  phasal  difference  intersect.  Oolitic  aggregates  often  give 
in  parallel  polarized  light  the  interference  figure  of  spherical  uniaxial 
aggregations  (PL  IX.  Fig.  2).  Calcite  does  not  show  pleochroism,  but 
the  strong  absorption  of  the  ordinary  ray  is  distinctly  noticed  when  the 
sections  are  not  too  thin. 

Sp.  gr.  =  2.Y2,  which  serves  to  distinguish  it  from  dolomite  and 
aragonite  (2.95).  Chemical  composition  =  CaO,  CO3 ;  it  is  easily  at- 
tacked by  acids.  It  is  distinguished  from  other  isomorphous  carbon- 
ates by  the  readiness  with  which  it  is  attacked  by  weak  acids  even 
at  ordinary  temperatures.  A  part  of  the  Ca  in  the  formula  may  be 
replaced  by  Mg,  Fe,  or  Mn,  without  affecting  the  solubility.  To  dis- 
tinguish magnesia-bearing  calcite  from  normal  calcite  it  is  advisable  to 
employ  the  method  of  G.  Linck  given  on  page  112. 

Calcite  frequently  contains  fluid  inclusions  and  rhombohedrons  of 
dolomite  or  magnesite.  Mechanical  deformation  is  recognized  .  by  the 
curving  of  the  cleavage  cracks  and  the  undulating  extinction,  and  is 
specially  common  in  granular  limestone. 

The  distribution  of  calcite  is  very  great,  even  when  we  leave  out  of 
consideration  its  prevalence  in  the  sedimentary  formations,  where  it 
occurs  as  marble,  limestone,  oolite,  chalk,  calcareous  tufa,  and  in  marl, 
calcareous  sandstones,  calcareous  clay  slate,  and  calcareous  mica- schist, 
'  etc.  In  all  kinds  of  eruptive  rocks,  more  especially  in  those  poor  in 
silica,  it  appears  as  the  filling  of  cavities  and  cracks,  and  partly  within 
the  compact  rock  mass.  It  is  very  often  a  product  of  atmospheric 
decomposition,  and  then  at  times  forms  complete  alteration  pseudo- 
morphs  after  lime  silicates  (plagioclase,  augite,  etc.),  or  replacement 
pseudomorphs  after  silicates  poor  in  lime  or  free  from  it  (olivine,  bio- 
tite,  etc.).  Moreover,  it  occurs  in  many  eruptive  rocks  (minettes,  ker- 


176  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS, 

santites)  in  apparently  primary  grains,  which  are  nevertheless  second- 
ary, occupying  the  spaces  once  filled  by  silicates.  In  other  cases  the 
presence  of  calcite  in  eruptive  rocks  is  due  to  infiltration  from  neigh- 
boring calcareous  rocks. 

Dolomite. 

In  contrast  to  calcite,  dolomite  occurs  in  rocks  chiefly  as  crystals, 
and  even  when  in  dense  homogeneous  aggregates  there  is  an  evident 
tendency  toward  outward  crystalline  boundaries,  so  that  it  assumes 
a  saccharoidal  structure.  The  form  of  the  crystals  occurring  in 
rocks  appears  to  be  almost  universally  the  fundamental  rhombohedron 
H  n  (1011),  seldom  more  acute  rhombohedrons.  Hence  the  cross-sec- 
tions are  triangular,  six-sided,  and  rhombic.  From  the  tendency  to 
curved  surfaces  which  characterizes  this  mineral,  the  outlines  are  often 
crooked,  bent,  and  distorted.  Twinning  has  not  been  observed  on 
rock-making  dolomite ;  the  lamination  parallel  to  —  \R  n  (0112), 
so  common  in  calcite,  is  wanting.  Oolitic  structure  occurs  with 
dolomite  as  with  calcite.  The  cleavage  parallel  to  R  n  (lOll)  is  just 
as  distinct  in  dolomite  as  in  calcite,  and  the  difference  in  the  rhombo- 
hedral  angles  of  both  substances  cannot  be  used  as  a  means  of  distin- 
guishing them  from  one  another 

The  optical  behavior  of  dolomite  is  the  same  as  that  of  calcite; 
the  character  of  the  double  refraction  is  negative ;  its  amount  is  consid- 
erable :  oona  —  1.68 17,  ena=1.5026  (Fizeau).  Interference  colors  and 
axial  figure  the  same  as  for  calcite.  In  transmitted  light  dolomite  is 
colorless  or  yellowish  to  brownish  in  consequence  of  the  alteration  of 
the  ferrous  carbonate  to  limonite  ;  it  is  gray,  brownish,  or  blackish 
through  organic  pigments.  Pleochroism  not  noticeable;  the  absorp- 
tion O  >  E. 

Sp.  gr.  varies  with  the  iron  percentage  from  2.85  to  2.95.  Chemi- 
cal com  position,  CaO,  CO2,  MgO,  CO2,  in  which  varying  amounts  of  Mg 
may  be  replaced  by  Fe  and  Mn.  Acetic  acid  and  cold  dilute  hydro- 
chloric acid  attack  dolomite  but  slightly ;  in  heated  hydrochloric  acid 
it  is  dissolved  rapidly  with  strong  effervescence. 

Dolomite  occurs  as  an  independent  rock  among  the  crystalline 
schists,  and  in  close  geological  connection  with  limestone  in  the  palaeo- 
zoic and  mesozoic  sedimentary  formations.  As  occasional  scattered 
crystals  it  is  found  in  limestones,  silicious  slates,  clay  slates,  and  phyl- 
lites,  especially  where  the  latter  show  regional  metamorphism.  It 
may  amount  to  an  essential  component  of  these  rocks. 


MA  GNESITE—BRE  UNNERITE—APA  TITE.  177 

Magnesite  and  Breunnerite. 

Magnesite  forms  suspended  crystals  in  the  form  of  the  fundamental 
rhombohedron  R  n  (1011),  when  it  occurs  as  an  accessory  constituent. 
When  it  is  an  essential  component  of  the  rock,  it  appears  as  isolated 
grains  or  in  granular  aggregations  without  crystalline  boundaries  to 
the  separate  grains.  Twinning  is  absent,  as  in  dolomite. 

The  cleavage  parallel  to  the  fundamental  rhombohedron  is  mani- 
fested, as  in  calcite  and  dolomite,  by  numerous  cracks  which  are  mostly 
straight,  less  frequently  slightly  curved. 

Its  behavior  toward  light  is  the  same  as  that  of  calcite  and  dolo- 
mite; the  double  refraction  is  strong  and  negative.  In  transmitted 
light  magnesite  is  colorless  to  grayish ;  also  yellowish  for  a  high  iron 
percentage. 

The  sp.  gr.  is  about  3.0-.  Chemical  composition,  MgO  CO2 ;  with 
the  introduction  of  the  isomorphous  iron  carbonate  in  variable  propor- 
tions it  passes  into  breunnerite.  The  corresponding  Ca  and  Mn  com- 
binations are  only  present  to  a  slight  extent.  Cold  hydrochloric  acid 
does  not  attack  magnesite  in  thin  sections  and  in  fragments. 

Breunnerite  often  becomes  yellow  to  reddish  brown  by  the  separat- 
ing out  of  limonite.  Magnesite  and  breunnerite  occasionally  occur  as 
accessory  minerals  in  chloritic  and  talcose  schists,  as  well  as  in  Swedish 
olivine  schists.  With  talc  they  form  certain  crystalline  schists,  and 
with  bronzite,  sagvandite. 

Apatite. 
Literature. 

F.  ZIEKEL,   Untersuclrangen  iiber  die    mikroskopische  Structur  und  Zusammen- 
setzung  der  Basal tgesteine.     Bonn.  1870.  73-74. 

In  eruptive  rocks  apatite  is  mostly  in  the  form  of  long,  slender 
hexagonal  columns  which  are  terminated  by  the  base  or  by  the  funda- 
mental pyramid  (1011 : 1011  =  80°  12'  to  80°  36'),  sometimes  by  both 
forms;  less  frequently  the  crystals  appears  as  short,  thick  columns 
bounded  by  the  same  faces.  The  latter  is  specially  noticeable  in  rocks 
of  the  gabbro  family.  On  the  other  hand,  in  the  crystalline  schists 
apatite  occurs  quite  often  in  rounded  or  long  oval  grains,  with  slight 
indications  of  crystalline  boundaries,  if  any.  Hence  sections  parallel 
to  the  base  are  regularly  hexagonal,  parallel  to  the  principal  axis  more 
or  less  elongated  rectangles,  which  are  sometimes  pointed  at  the  ends 
or  have  the  corners  truncated,  or  the  cross-sections  may  be  round  or 


178  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

oval.  Crystals  occur  grown  together  in  parallel  position,  but  twinning 
is  absent.  The  cleavage  parallel  to  the  base  and  prism  is  seldom  ob- 
served microscopically,  but  the  long  columnar  crystals  almost  always 
exhibit  a  transverse  jointing  so  that  they  fall  into  distinct  pieces  which 
not  infrequently  have  been  more  or  less  dislocated. 

Apatite  of  itself  is  clear  and  transparent,  but  sometimes  in  rocks 
possesses  a  gray,  violet-blue,  yellowish  or  brownish  color  of  different 
intensity.  Its  index  of  refraction  is  higher  than  that  of  the  other 
colorless  minerals  generally  associated  with  it ;  hence  its  bright  white 
color,  and  not  inconsiderable  relief.  The  double  refraction  is  weak  and 
negative.  In  apatite  from  Jumilla,  Spain,  cona  =  1.6388,  ena  ==  1.6346 
(Lattermann).  Therefore  the  interference  colors  in  thin  section  scarcely 
exceed  white  of  the  1st  order,  being  mostly  in  the  grayish-blue  tones. 
In  convergent  light  basal  sections  give  only  a  cross,  without  rings. 
Colorless  apatite  has  no  pleochroism ;  the  colored  apatite  is  always 
distinctly  and  often  strongly  pleochroic,  the  absorption  being  E  >  O, 
which  is  a  convenient  microscopical  means  of  its  determination  from 
tourmaline.  This  strong  absorption  of  the  extraordinary  ray  is  notice- 
able by  careful  observation  even  in  colorless  apatite.  The  optical 
anomalies  frequently  noticed  in  large  attached  crystals  are  scarcely  ever 
observed  in  the  microscopical  individuals  found  within  rocks. 

On  account  of  its  high  specific  gravity,  3.16-3.22,  apatite,  when 
separated  mechanically  from  the  rocks,  falls  with  the  minerals  having 
heavy  metallic  bases,  and  may  be  generally  separated  from  these  with 
the  electro-magnet  without  trouble.  From  non-magnetic  minerals 
(zircon,  titanite,  rutile,  etc.)  it  may  be  separated  by  means  of  Klein's 
solution. 

Chemical  composition  —  3Ca8P3OB  +  Ca  (Cl,  Fl)a.  It  js  readily 
soluble  in  acids;  from  the  solution  upon  the  addition  of  ammonium 
molybdate  there  is  precipitated  yellow  octahedral  or  rhombic  dodeca- 
hedral  crystals  or  groups,  which  are  formed  even  in  the  cold  (PL 
XIII.  Fig.  5).  In  another  part  of  the  solution  dilute  sulphuric  acid 
precipitates  crystals  of  gypsum.  Many  crystals  contain  a  noticeable 
percentage  of  manganese ;  the  solution  of  these  upon  treatment  with 
hydrofluosilicic  acid  yields  rhombohedral  crystals  with  prismatic  habit 
of  fluosilicate  of  manganese.  Although  apatite  is  so  easilv  attacked 
by  acids,  it  is  remarkable  that  it  is  found  perfectly  fresh  in  rocks  which 
are  completely  decomposed.  In  some  instances  it  is  of  ideally  pure 
substance,  in  others  it  is  more  or  less  filled  with  interpositions,  of 
which  gas  and  fluid  inclusions  predominate,  while  glass  inclusions  are 
rarer.  These  are  often  arranged  in  a  very  orderly  manner,  and  mostly 


NEPHELINE  AND  EL^EOLITE.  179 

show  a  central  accumulation  parallel  to  the  principal  axis  ;  in  other  cases 
they  are  in  concentric  shells  parallel  to  the  outward  form  of  the  crystal, 
or  may  be  scattered  generally  through  the  whole  mineral.  Very  rarely 
the  interpositions  are  massed  peripherally  or  are  arranged  parallel  to 
the  principal  vertical  section,  so  that  they  form  six-rayed  stars  in  cross- 
sections.  The  surface  of  the  apatite  is  often  rough  through  corrosion, 
and  covered  with  irregular  depressions. 

Apatite  is  present  in  all  rocks,  and  in  the  eruptive  rocks  appears  as 
one  of  the  oldest,  if  not  the  oldest  secretion  of  the  magma.  The 
needles  of  this  mineral  pass  uniformly  through  all  the  other  constitu- 
ents. Though  mostly  disseminated  quite  uniformly  throughout  the 
whole  rock  mass,  it  is  sometimes  crowded  together  with  the  older 
secretions  (iron  ores,  zircon,  mica).  It  appears  to  be  more  abundant 
in  the  older  granular  eruptive  rocks  and  in  the  feldspathic  crystalline 
schists,  than  in  the  younger  eruptive  rocks  and  in  the  feldsparless 
schists ;  the  basic  eruptive  rocks  also  appear  to  contain  more  apatite 
than  the  acid  ones.  It  is  found  particularly  associated  with  biotite 
and  nepheline. 

Neplieline  and  Elceolite. 
Literature. 

H.  &OSENBUSCH,  Der  Nephelinit  vom  Katzenbuckel.  Freiburg  i.  B.  1869.  46-59. 
F.  ZIRKEL,  Mikroskopische  Untersuchungen  liber  die  Zusammensetzung  und  Struc- 

tur  der  Basaltgesteine.     Bonn.  1870.  38. 
—  Ueber  die  mikroskopische  Zusammensetzung  der  Phonolitne.     Pogg.  Ann.  1867. 

CXXXI.  303. 

Nepheline  and  elaeolite  bear  the  same  relation  to  one  another  as 
sanidine  and  orthoclase  do.  The  first  includes  the  glassy  colorless 
occurrences  in  the  younger  volcanic  rocks ;  the  second  the  massive 
occurrences,  often  somewhat  colored,  in  the  older  plutonic  rock  and 
their  pegmatitic  secretions.  Identical  in  substance  and  in  all  essential 
physical  characters,  they  are  nevertheless  rightly  separated  on  account 
of  their  diverse  habit  and  different  geological  position. 

Nepheline  and  elseolite  show  themselves  in  the  rocks  partly  as 
completely  developed,  short  prismatic  crystals  of  the  form  oo  P 
(1010) .  oP  (0001),  whose  basal  edges  are  sometimes  truncated  by  small 
faces  of  P(1011).  The  angle  over  the  edge  of  the  prism  is  88°  10'. 
The  nepheline  individuals  in  general  are  considerably  smaller  than 
those  of  elseolite  when  compared  with  the  grains  of  the  containing  rock. 
But  the  elseolite  indivduals  also  sink  to  microscopic  dimensions.  The 


180  PHYSIOGRAPHY  OF  THE  HOCK-MAKING  MINERALS. 

outline  of  the  sections  are  naturally  hexagonal  parallel  to  the  base,  and 
short  rectangular  to  quadratic  in  longitudinal  sections,  occasionally 
with  the  corners  truncated.  An  outward  crystalline  boundary  is  often 
completely  wanting  in  elaeolite,  as  would  naturally  be  the  case  in 
granular  rocks  ;  but  nepheline  seldom  occurs  massive.  Their  bound- 
ary in  this  case,  then,  is  in  no  way  characteristic  of  the  minerals. 

The  cleavage  parallel  to  oo  P  (1010)  arid  oP  (0001)  is  seldom  ob- 
served under  the  microscope  in  glassy  nepheline ;  it  is  more  common 
in  elaeolite.  The  cleavage  becomes  more  evident  after  decomposition 
has  attacked  these  minerals,  as  the  alteration  products  are  first  de- 
posited along  the  cleavage  cracks. 

Nepheline  and  elaeolite  become  transparent  and  colorless;  their  in- 
dex of  refraction  corresponds  very  closely  to  that  of  Canada  balsam, 
and  the  double  refraction  is  weak.  Hence  the  absence  of  relief  in  thin 
section  and  their  low  interference  colors  (grayish  blue  or  at  most  white 
of  the  1st  order).  The  low  index  of  refraction  may  be  used  as  a  means 
of  distinguishing  these  minerals  from  apatite.  The  character  of  the 
double  refraction  is  negative.  In  nepheline  from  Vesuvius,  J.  E, 
Wolff  determined  ena  =  1.5376,  6^  =  1.54:16;  M.  E.  Wadsworth, 
e^  =  1.5378,  cona/  =  1.54:27 ;  in  elaeolite  from  Hot  Springs,  Arkansas, 
S.  L.  Penfield  found  ena  =  1.54:22,  cona  —  1.54:69.  In  convergent  light 
thin  sections  give  a  broad  interference  cross  without  rings.  In  parallel 
light  the  double  refraction  of  very  microscopic  individuals  is  not 
at  all  noticeable,  except  by  using  a  gypsum  plate  or  quartz  wedge. 

Sp.  gr.  =  2.55-2.61;  it  is  generally  somewhat  higher  for  elseolite 
than  for  nepheline,  probably  on  account  of  the  difference  in  their  inter- 
positions. It  lies  between  that  of  the  triclinic  and  orthorhombic  feld- 
spars, and  permits  their  mechanical  separation.  The  chemical  com- 
position is  4Na2O,  4A12O3,  9SiO2,  in  which  one  quarter  of  the  Na  is 
generally  replaced  by  K,  while  only  a  very  small  amount  of  Ca  occurs 
in  these  minerals.  Nepheiine  and  elaeolite  gelatinize  quite  easily  and 
quickly  with  hydrochloric  acid,  but  more  difficultly  than  the  minerals 
of  the  sodalite  group.  Thisgelatinization  and  the  method  of  staining  it 
already  described  (p.  65)  are  the  best  means  of  recognizing  and  deter- 
mining nepheline  and  elaeolite  under  the*  microscope.  The  absence  of 
calcium  in  the  solution  prevents  a  confusion  with  melilite. 

Elaeolite  very  commonly  carries  microscopic  interpositions  of  angite 
and  hornblende  needles,  fluid  and  gas  inclusions.  In  the  fluid  inclu- 
sions cubes  of  NaCl  are  sometimes  secreted.  Nepheline  is  also  rich  in 
inclusions  of  the  minutest  dimensions,  which  are  often  scarcely  deter- 
minable;  they  are  augite  microlites,  fluid,  gas,  and  glass  inclusions.  In 


CASCRIN1TE.  181 

both  minerals  the  arrangement  of  the  interpositions  is  mostly  in  zones 
{PL  XYI.  Fig.  6,  and  PI.  XVII.  Fig.  1) ;  they  are  seldom  crowded 
together  at  the  centre. 

Elseolite  and  nepheline  are  easily  altered  to  zeolites,  of  which 
natrolite  appears  most  frequently  to  form  pseudomorphs  after  them. 
The  process  commences  from  the  cracks  and  margin,  and  leads  to  the 
formation  of  parallelly  fibrous,  confusedly  fibrous  or  radiating  aggre- 
gates, with  brilliant  double  refraction.  Nepheline  and  elseolite  are  also 
known  to  alter  into  analcite  and  thomsonite. 

While  the  zeolitization  of  these  minerals  has  taken  place  through 
the  action  of  hot  waters  soon  after  the  solidification  of  the  rock,  there 
is  produced  from  them  through  the  ordinary  atmospheric  influences, 
muscovite  and  kaolin  (liebenerite  and  gieseckite). 

Nepheline  is  only  known  in  volcanic  rocks ;  it  occurs  with  s'anidine 
in  phonolites*  and  leucite  porphyries,  with  triclinic*  feldspars  in  tes- 
chenites  and  tephrites,  without  feldspars  in  nepheline  basalts  arid 
nephelinites,  and  with  leucite  in  leucite  basalts  and  leucitites. 

Eiseolite  occurs  with  orthoclase  as  an  essential  ingredient  of  elseo- 
lite syenite,f  and  occurs  in  a  rock  without  feldspars  at  Mt.  Jivaara  in 
Finland.  It  is  an  accessory  mineral  in  the  augite  syenites  of  Southern 
Norway.  The  frequent  occurrence  of  nepheline  and  elseolite  with 
minerals  of  the  sodalite  group  is  to  be  noted. 

In  all  the  eruptive  rocks  the  formation  of  nepheline  and  elseolite 
follows  the  secretion  of  the  bisilicates  and  the  micas,  and  in  the  first 
generation  at  least  precedes  that  of  the  feldspars.  When  minerals  of 
thd  sodalite  group  occur  with  them  as  primary  crystals  they  are  the 
older  secretions. 

Eucryptite  is  a  lithia  nepheline  described  by  Brush  and  E.  S. 
DanaJ  as  an  alteration  product  of  spodumene  from  Branch ville,  Conn. 

Cancrinite. 
Literature. 

A.   KOCH,   Petrographische  und  tektonische  Verhaltnisse  des  Syenitstockes  von 

Ditro  in  Ostsiebenbiirgen.     K  J.  B.  B.-B.  I.  1881.  144. 
H.  RAUFF,  Ueber  die  chemische  Zusammensetzung  des  JSTephelins,  Cancrinits  und 

Mikrosommits.     Z.  X.  1878.  II.  456-468. 
A.  E.  TORNEBOHM,  Om  den  s.  k.  Fonoliten  fran  Elfdalen,  dess  klyftort  och  fore- 

komstadt.     Geol.  Foren.  i.  Stockholm  Forhdl.  1883.  VI.  No.  80.  383. 

*  Am.  Journ.  1880,  XX.  259. 

f  J.  H.  Caswell  has  described  phonolite  from  the  Black  Hills,  Dakota.  Microscopic 
Petrography  of  the  Black  Hills  of  Dakota.  Washington,  1876.  492. 

}  Elseolite  syenite  occurs  near  Deckertown,  N".  J.  (B.  K.  Emerson,  Am.  Journ.  Sci., 
April  1882.  302),  also  at  Litchfield,  Me.,  and  Magnet  Cove,  Ark. 


182  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

Cancrinite  as  a  rock-making  mineral  sometimes  occurs  in  long- 
columnar  crystals  with  the  faces  ooP  (1010),  P  (1011) ;  more  frequently 
in  staff-like  individuals  developed  only  in  the  prism  zone;  occasionally 
it  is  in  irregular  grains  whose  outlines  are  dependent  on  the  other  rock 
constituents. 

The  cleavage  parallel  to  ccP  (lOlu)  appears  microscopically  in 
distinct  and  sharp  cracks,  both  in  transverse  and  longitudinal  sections ; 
longitudinal  sections  also  exhibit  a  distinct  cleavage  parallel  to  oP 
(0001).  An  imperfect  cleavage  appears  to  run  parallel  to  GO.P2  (1120). 

Cancrinite  when  fresh  is  transparent  and  colorless.  The  index  of 
refraction  is  lower  than  that  of  Canada  balsam,  the  double  refractron 
is  negative,  and  considerably  stronger  than  that  of  nepheline.  The 
interference  colors  in  thin  section  generally  range  from  orange  of  the 
1st  order  upwards,  and  are  similar  to  those  of  scapolite.  In  cancrinite 
from  Miask,  ep  ='1.4955,  G?P  =  1.5244  (Osann).  Cross-sections  in  con- 
vergent light  give  a  sharp  cross  with  rings. 

The  sp.  gr.  is  about  2.45,  which  greatly  facilitates  its  mechanical 
separation  from  the  minerals  associated  with  it.  The  chemical  com- 
position =  4lSTa,aO,  4AlaO8,  9SiO2  +  2CaO ,  CO3  +  3H.O.  It  is  de- 
composed by  cold  hydrochloric  acid  with  the  separation  of  gelatinous 
silica  and  the  liberation  of  bubbles  of  carbonic  acid.  When  heated  to 
redness  in  thin  section  it  becomes  clouded,  and  may  thus  be  distin- 
guished from  nepheline.  Cancrinite  has  no  constant  microstructnre  : 
it  is  sometimes  quite  free  from  interpositions  ;  at  others  it  has  the  same 
inclusions  as  elseolite,— especially  the  pyroxene  needles,  which  are 
arranged  with  their  longitudinal  axes  parallel  to  the  principal  axis 
of  the  cancrinite.  The  red  and  yellow  color  of  many  occurrences  arises 
from  the  interposition  of  plates  of  hematite.  The  process  of  alteration 
is  similar  to  that  in  elseolite. 

Cancrinite  is  only  known  as  yet  in  elseolite  syenites  (Miask,  Brevig, 
Lichfield)  in  company  with  elaeolite  and  sodalite. 

Tourmaline. 

In  many  rocks  tourmaline  forms  perfectly  developed  columnar 
crystals,  often  with  very  distinct  hemimorphism :  on  one  pole  there  is 
usually  R,  n  (1011)  only,  with  the  terminal  angle  133°  10'-133°  20', 
rarely  with  derived  rhombohedrons ;  on  the  other  pole  is  the  base  ;  iruthe 
vertical  zone  there  is  sometimes  only  00^2  (1120),  sometimes  the  three- 
sided  00 7?  (1010)  also.  Thus  the  cross-sections  are  regular  hexagons, 
or  hexagons  with  alternately  truncated  corners,  the  longitudinal  sec- 


TOURMALINE.  183 

lions  being  lath-shaped.  More  frequently,  however,  tourmaline  appears 
in  staff-like  individuals  without  sharp  crystalline  development,  in 
bunched  or  finely  radiating  aggregates ;  the  cross-sections  of  separate 
individuals  then  are  irregularly  rounded.  More  rarely  it  assumes  the 
form  of  irregular  grains.  A  shelly  structure  is  quite  common,  the 
difference  in  color  of  the  kernel  and  shells  clearly  indicating  an  iso- 
morphous  lamination.  Less  frequently  the  layers  vary  horizontally ; 
and  still  more  rarely  they  assume  the  form  of  an  axial  cross,  differing 
in  color  from  the  main  mass  of  the  crystal. 

Cleavage  is  not  recognizable  microscopically,  but  irregular  trans- 
verse and  longitudinal  cracks  are  very  common,  especially  in  the  larger 
individuals.  The  tourmaline  occurring  in  rocks  is  never  perfectly 
colorless,  but  is  always  transparent  and  colored.  Moreover,  the  colors 
vary  extraordinarily  both  in  kind  and  intensity.  Yellow,  brown, 
green,  red,  and  especially  violet  blue,  are  those  which  most  frequently 
appear  in  transmitted  light.  The  index  of  refraction  is  moderate,  the 
double  refraction  quite  strong  and  negative.  Des  Cloizeaux  found  in  a 
crystal  consisting  of  blue  and  green  shells,  for  both  colors,  ep  —  1.6240, 
cop  =  1.6444;  Miklucho-Maclay  found  in  a  colorless  tourmaline  from 
Elba,  ena  =  1.6208,  wna  —  1.6397.  The  relief  of  the  section  against 
the  colorless  rock  constituents  is  distinctly  perceptible ;  the  surface  is 
noticeably  rough.  Cross-sections  give  a  sharp  interference  cross  with 
clearly  determinable  negative  character.  The  pleochroism  is  stronger 
as  the  color  is  deeper,  but  is  distinctly  noticeable  even  in  quite  light- 
colored  individuals.  The  absorption  is  always  strong  for  the  ordinary 
ray  and  weak  for  the  extraordinary  ray.  The  colors  change  with  the 
body  color.  Similarly  strong  differences  of  absorption  are  only  ex- 
hibited in  rocks  by  dark  mica,  hornblende,  and  allanite ;  the  first  two 
are  easily  distinguished  by  their  cleavage,  while  the  third  is  only  dis- 
tinguished by  its  crystallization. 

Optical  anomalies  are  rarely  perceptible.  They  are  only  shown  in 
the  cross-sections,  and  especially  in  convergent  light  through  the  sepa- 
ration of  the  interference  cross  into  hyperbolas  with  very  varying  in- 
tervals between  their  poles.  Apparently  it  is  those  individuals  com- 
posed of  isomorphous  layers  which  most  frequently  possess  these 
anomalies. 

The  sp.  gr.  varies  with  the  chemical  composition  from  3.0-3.24, 
rising  with  the  increase  of  the  bivalent  metals.  The  chemical  compo- 
sition is  a  very  variable  one,  and  according  to  Rammelsberg's  investi- 
gations may  be  explained  as  an  isomorphous  mixture  of  the  mole- 
cules : 


184  PHYSIOGRAPHY  OF  TUB  ROCK-MAKING  MIJXB11ALS. 

NaHO,  BA,  3A1A,  4SiO,, 
5MgO,  BA,  A1A,  5Si02, 
5FeO,  BA,  A1A,  5SiO, ; 

in  which  in  the  first  molecule,  in  place  of  Na,  K  and  Li  may  also  occur, 
and  in  the  second  and  third,  in  the  place  of  Mg  and  Fe,  Mn  may  occur. 
Tourmaline  is  not  acted  on  by  acids,  including  hydrofluoric,  and  may 
therefore  be  easily  separated  from  the  rocks  by  chemical  means,  even 
in  small  quantities.  It  is  then  obtained  together  with  rutile,  spinel, 
garnet,  zircon,  andalusite,  sillimanite,  etc.,  and  may  be  separated  from 
these  substances  by  means  of  Klein's  solution,  supplemented  by  mag- 
netic methods. 

Tourmaline  possesses  no  distinctive  microstructure  which  can  be 
used  in  its  diagnosis.  It  occasionally  encloses  part  of  the  minerals 
associated  with  it,  or  gas  and  fluid  inclusions  besides  liquid  carbon  di- 
oxide, but  not  with  any  degree  of  constancy. 

As  an  accessory  constituent,  tourmaline  occurs  in  the  older  granular 
and  acid  eruptive  rocks  (granite,  syenite,  diorite).  In  massive  occur- 
rences of  these  rocks  it  is  situated  most  usually  on  the  periphery  and  in 
the  vicinity  of  fissures  and  veins.  When  these  rocks  form  dikes  it  is 
more  often  scattered  through  the  whole  mass.  It  is  particularly  fre- 
quent in  the  "  greisen"-like  modifications  of  granite,  and  may  in  these 
cases  be  easily  confused  with  cassiterite,  when  it  forms  granular  or  radial 
aggregates.  It  may  be  distinguished  from  it  in  cross-sections  by  test- 
ing the  optical  character  of  the  interference  cross  with  the  quarter 
undulation  mica  plate,  and  in  longitudinal  sections  by  means  of  the 
quartz  wedge.  It  is  also  very  common  in  the  contact  zones  of  schists 
near  granites,  and  may  become  so  abundant  locally  as  to  form  tour- 
maline hornstone,  as  in  many  places  in  Cornwall,  and  especially  in 
the  White  Mountains  at  Mt.  Willard,  N.  H.*  It  very  rarely  occurs  in 
the  mesozoic  pyrophritic  acid  eruptive  rocks  (quartz  porphyries  and 
quartz  porphyrites) ;  it  is  almost  completely  absent  from  the  equiva- 
lent tertiary  lavas.  Its  whole  mode  of  occurrence  in  eruptive  rocks 
and  their  contact  zones  indicates  that  it  was  not  directly  secreted  out 
of  the  eruptive  magma,  but  resulted  from  the  action  of  fumaroles 
carrying  fluorine  and  boron  on  the  eruptive  rock,  especially  on  its 
feldspar  and  mica.  Tourmaline  is  very  common  as  isolated  crystals, 
mostly  very  sharply  defined,  in  the  quartz  and  feldspar-bearing  mem- 
bers of  the  crystalline  schists,  gneisses,  granulites,  etc.,  as  well  as  in 


*  G.  W.  Hawes,  The  Albany  Granite  and  its  Contact  Phenomena.      Amer.  Jour. 
1881.  XXI.  21-32. 


EUDIALTTE—  CHLORITE  GROUP.  185 

phyllites  and  clay  slates  (PL  XY.  Fig.  4).  Owing  to  its  resistance  to 
decomposition,  tourmaline  remains  unaltered  in  the  detritus  of  these 
rocks  and  passes  into  the  composition  of  clastic  rocks. 

Eudialyte. 

Eudialyte  forms  either  crystals  of  very  variable  dimensions,  which, 
however,  seldom  become  wholly  microscopic  and  exhibit  predomi- 
dantly  the  forms^  oR,  (0001), >  R,  n  (1011),  —  f#,  n  (1012),  while  the 
forms  \R,  n  (1014),  o>7?,  (1010)  and  ooP2  (1120)  are  subordinate ;  or 
it  forms  grains  with  incomplete  crystallographic  boundaries.  The  ter- 
minal angle  of  R  is  73°  30'. 

The  cleavage  parallel  to  oR  (0001)  is  distinctly  perceptible  micro- 
scopically, while  the  incomplete  cleavage  parallel  to  \R,  n  (1014)  and 
oojP2  (1120)  is  not  recognizable. 

Eudialyte  becomes  transparent  with  light  yellowish-red  and  purplish 
blood-red  color ;  possesses  quite  strong  positive  double  refraction,  and 
a  high  index  of  refraction,  as  the  decided  relief  and  rough  surface  in 
thin  section  indicates.  The  pleochroism  is  weak.  The  whole  appear- 
ance suggests  garnet. 

The  sp.  gr.  varies  from  2.84-3.05.  The  chemical  composition  ap- 
proaches Na2O,  2(Ca,  Fe)O,  6(Si,  Zr)O2.  The  chlorine  percentage  of 
the  analyses  is  referred  to  inclusions  of  sodalite,  which  are  sometimes 
recognized  microscopically.  With  hydrochloric  acid  eudialyte  gelatin- 
izes quite  easily. 

The  microscopic  individuals  are  usually  free  from  inclusions ;  the 
larger  massive  grains  occasionally  enclose  elaeolite,  sodalite,  arfvedsonite, 
and  numerous  fluid  inclusions  of  rhombohedral  or  rounded  form, 
which  are  quite  large  and  are  arranged  in  straight  lines. 

Eudialyte  is  often  a  prominent  constituent  of  Greenland  elseolite 
syenite,  from  which  Yrba  described  it ;  he  also  found  it  accessory  to 
similar  rocks  from  the  islands  of  Langesundf jord  in  Southern  Norway. 

Eucolite  is  chemically  and  crystallographically  identical  with 
eudialyte  ;  it  is  distinguished,  however,  from  the  latter  by  the  distinct 
cleavage  parallel  to  o>P2  (1120),  and  the  negative  character  of  the 
double  refraction.  This  mineral  also  is  found  in  the  elseolite  syenites 
of  Southern  Norway. 

Chlorite  Group. 

In  the  chlorite  group  have  been  placed  a  number  of  minerals  which 
show  a  relationship  because  of  their  chemical  composition,  their  crystal- 
lographic development,  physical  properties,  and  of  their  whole  habit  and 


186          PHYSIOGRAPHY  OF  THE  HOCK-MAKING  MINERALS. 

geological  value,  but  which,  however,  partly  on  account  of  their  opti- 
cal behavior,  partly  because  of  their  chemical  composition,  have  been 
divided  into  the  three  varieties :  pennine,  clinochlor,  and  ripidolite. 
The  determination  of  these  varieties,  even  in  comparatively  well-devel- 
oped crystals,  is  not  without  difficulties,  and  in  rock- making  occur- 
rences often  becomes  impossible. 

Since  the  subdivisions  of  this  group  in  present  use  are  very  probably 
provisional,  the  name  chlorite  will  be  used  to  embrace  all  the  minerals 
of  the  chlorite  group.  It  has  only  been  placed  in  the  hexagonal 
crystal  system  because  the  observations  applicable  to  most  cases  practi- 
cally lead  to  this  system.  Nevertheless,  it  is  highly  probable  that  all 
that  is  here  described  as  chlorite  is  to  be  referred  in  fact  to  the  mono- 
clinic  system.  Rock-making  chlorite  appears  mostly  in  the  form  of 
flat  leaves  of  variable  size,  or  of  somewhat  dense  scales  of  irregular 
outline.  The  scales  generally  lie  with  their  faces  upon  one  another  in 
parallelly  laminated  aggregations ;  less  frequently  they  arrange  them- 
selves spirally  in  rosettes.  When  a  crystallographic  outline  is  observed 
on  the  scales  or  plates  it  is  most  frequently  hexagonal,  rarely  triangular, 
or  irregularly  polygonal,  as  though  the  hexagonal  prisms  of  the  first 
and  second  order  had  been  developed  with  an  incomplete  number  of 
faces.  The  cross-sections  of  these  laminated  aggregates  are  more  or  less 
lath-shaped,  often  with  curved  and  imperfectly  parallel  edges.  Another 
development  of  chlorite  is  the  fibrous,  in  which  the  fibres  are  some- 
times parallel,  sometimes  divergent,  and  at  times  grouped  in  uniformly 
radiating  spherulitesi  Lastly,  chlorite  frequently  occurs  in  the 
minutest  particles,  which  exhibit  neither  laminated  nor  fibrous  struc- 
ture. This  is  usually  the  case  when  chlorite  occurs  as  finely  divided 
pigments  in  other  minerals. 

Chlorite  cleaves  very  perfectly  parallel' to  the  flat  face,  which  is 
considered  as  the  basal  plane ;  the  cleavage  plates  are  generally  flexi- 
ble. In  cross-sections  the  cleavage  manifests  itself  by  numerous  lines 
running  parallel  to  the  edge  of  the  section,  which  are  often  twisted  in 
the  same  manner  as- this  is.  The  perfection  of  the  cleavage  microscopi- 
cally is  scarcely  second  to  that  of  mica.  Plates  parallel  to  the  cleavage 
face  never  show  cleavage  lines — a  fact  which  may  be  used  to  distinguish 
it  microscopically  from  chloritoid. 

Chlorite  is  generally  green  by  incident  and  by  transmitted  light, 
although  the  depth  of  color  varies  from  greenish  white  to  dark  green. 
The  index  of  refraction  is  low ;  the  double  refraction  is  very  weak. 
Haidinger  determined  on  pennine  e  =  1.575,  GO  =  1.576 ;  Des  Cloizeaux, 
e  p=  1.576,  cop  =  1.577.  The  character  of  the  double  refraction  varies, 


CHLORITE  GROUP.  187 

and  has  been  found  even  for  the  same  occurrence  sometimes  positive, 
sometimes  negative.  Plates  parallel  to  the  cleavage  face  usually  show 
themselves  quite  isotropic  in  parallel  polarized  light,  or  when  rotated 
between  crossed  nicols  exhibit  only  a  very  slight  illumination,  which 
often  appears  to  be  due  to  the  fact  that  the  plates  are  not  lying  per- 
fectly flat.  In  convergent  light  they  generally  give  an  indistinctly 
defined  interference  cross,  between  whose  arms  the  light  quadrants  are 
scarcely  visible.  The  centre  of  the  cross  of  ten  liesexcentrically  ;  more- 
over, it  not  infrequently  opens  into  two  hyperbolas  with  very  variable 
polar  interval.  These  phenomena  indicate  monoclinic  chlorite  (clino- 
chlore  or  ripidolite).  Cross-sections  of  plates  or  fibres  of  chlorite  show 
themselves  as  doubly  refracting,  but  mostly  with  very  low  interference 
colors ;  often  the  double  refraction  is  only  noticeable  by  careful  obser- 
vation, even  for  light-colored  varieties.  The  interference  colors  in  thin 
section  do  not  exceed  white  of  the  first  order,  but  these  are  difficultly 
distinguishable  because  of  the  proper  color  of  the  mineral,  and  often 
combine  with  the  latter  to  form  a  peculiar  blue.  The  extinction  lies 
apparently  parallel  to  the  cleavage,  or  deviates  but  very  slightly  from 
it ;  consequently  in  the  actually  monoclinic  chlorites  the  bisectrix  must 
stand  very  nearly  perpendicular  to  the  base.  This  is  also  a  good 
criterion  in  distinguishing  it  from  most  chloritoids.  Twinning  has  not 
been  observed  in  rock-making  chlorite.  Radially  fibrous  aggregates 
give  the  interference  cross  of  spherulites ;  the*  arms  lie  apparently 
parallel  to  the  principal  sections  of  the  nicols.  Chlorite  is  distinctly 
pleochroic,  and  in  the  same  manner,  both  in  the  apparently  hexagonal 
and  in  the  monoclinic  varieties,  O  is  green,  E  pale  yellow  to  red  or 
brown.  The  difference  between  the  two  rays  is  more  perceptible  as 
the  chlorite  is  deeper  colored.  The  pleochroism  is  generally  notice- 
able, even  when  the  double  refraction  can  only  be  observed  by  the  in- 
sertion of  sensitive  plates. 

The  specific  gravity  varies  between  2.65  and  2.97  with  increasing 
iron  percentage.  The  chlorite  minerals  are  considered  as  isomorphous 
mixtures  of 

2HaO,  3(M<?,  Fe)O,  2SiO2 
and  2H,0,  2(Mg,  Fe)6,  (Al,  Fe)2O3,  SiO2, 

in  the  proportion  of  3 : 2  to  1:2.  All  chlorites  gelatinize  in  thin  section* 
with  hot  hydrochloric  acid,  often  even  with  cold ;  still  more  easily  witli 
sulphuric  acid,  and  may  then  be  colored.  Besides  magnesia  and  ironr 
alumina  may  also  be  abundantly  detected  in  the  solution  by  the  methods 
already  given.  Upon  being  heated  to  redness  in  thin  section  on  plati- 


188          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

num  foil  chlorite  loses  its  water  and  becomes  opaque.     Ferruginous 
varieties  are  colored  reddish  brown  to  black  in  this  way. 

The  chlorites  are  among  the  most  widely  disseminated  substances  in. 
rocks,  but  in  eruptive  rocks  and  their  tufas  they  are  always  secondary 
formations  and  products  of  alteration.  They  are  derived  from  the 
aluminous  members  of  the  biotite  and  phlogopite  micas,  of  the  pyroxene 
and  amphibole  families,  and  of  garnets.  To  them  many  eruptive  rocks 
owe  their  green  color.  This  secondary  origin  of  the  chlorites  is  un- 
doubtedly proved  by  their  occurrence  as  pseudomorphs  after  the  above- 
mentioned  minerals,  often  with  the  complete  preservation  of  the  forms 
of  the  original  substance.  In  other  cases  chlorite  is  formed  by  magnesia- 
and  iron-bearing  solutions  from  non-aluminous  substances  acting  upon 
solutions  from  aluminous  minerals  which  are  free  from  magnesia  and  iron. 
In  this  way  "  replacing"  pseudomorphs  are  formed,  for  example,  after 
feldspar.  Chlorite  has  a  further  distribution  in  the  schistose  rocks:  it 
occurs  in  chlorite  schists  as  the  only  essential  constituent,  and  is  here 
accompanied  by  amphibole,  magnetite,  dolomite,  etc.,  as  accessory 
minerals.  Together  with  amphibole,  it  appears  in  many  amphibolites, 
especially  those  of  phyllite  formations  ;  with  mica  in  mica  schists  and 
phyllite ;  with  feldspar  and  mica  in  phyllite  gneisses ;  with  epidote, 
augite,  and  actinolite,  besides  feldspar,  and  quartz  in  the  so-called  green 
schists.  It  also  appears  in  the  phyllites  proper  and  in  clay  slates  as 
quite  a  regular  and  often  a  very  abundant  constituent. 


ORTHORHOMBIC  MINERALS.  189 


MINERALS  OF  THE  ORTHORHOMBIC  SYSTEM. 

SECTIONS  of  the  regularly  bounded  minerals  belonging  to  the  ortho- 
rhora'bic  system  or  the  figures  made  by  their  cleavage  cracks  are  bisym- 
metric  when  they  lie  parallel  to  two  axes,  monosymmetric  when  they 
are  parallel  to  only  one  axis,  and  asymmetric  when  they  intersect  all 
three  axes.  If  the  pyramidal  faces  or  cleavage  planes  are  wanting 
the  sections  may  apparently  possess  a  higher  symmetry  than  they  actu- 
ally do. 

Pinacoidal  cleavage  furnishes  parallel  systems  of  lines  in  all  sections 
which  are  not  parallel  to  the  cleavage  itself  ;  prismatic  cleavage  or  that 
parallel  to  two  pinacoids  gives  parallel  cracks  in  all  sections  lying  in  the 
zones  of  these  cleavages,  and  intersecting  cracks  in  all  other  sections  ; 
cleavage  parallel  to  three  pinacoids  or  to  a  pyramid  gives  in  all  sections 
figures  which  are  enclosed  by  intersecting  lines. 

The  ellipsoid  of  elasticity  of  orthorhombic  crystals  is  triaxial,  and 
its  three  axes  coincide  with  the  axes  of  the  crystal.  Therefore  the 
plane  of  the  optic  axes  always  lies  in  a  principal  crystallographic  sec- 
tion, and  the  optic  axes  for  light  of  different  wave-lengths  are  disposed 
symmetrically  with  respect  to  two  of  the  crystallographic  axes.  If  the 
bisectrix  of  the  acute  angle  of  the  optic  axes  is  the  direction  of  least 
elasticity  (c),  the  crystal  is  called  positive  ;  it  is  said  to  be  negative  when 
the  axis  of  greatest  elasticity  (a)  bisects  the  acute  angle.  Sections  per- 
pendicular to  an  optic  axis  are  uniformly  light  in  all  positions  in 
parallel  light  between  crossed  nicols,  and  yield  in  convergent  light  an 
axial  figure  in  the  centre  of  the  field  of  view.  This  figure  consists  of 
approximately  circular,  concentric  curves  cut  by  a  dark  bar,  which  is 
always  straight  when  the  axial  plane  coincides  with  a  principal  plane 
of  the  nicols.  Upon  rotating  the  section  in  its  plane  the  bar  turns  and 
curves  slightly  to  an  hyperbola.  All  other  sections  become  light- 
colored  and  dark  four  times  during  a  rotation  in  parallel  white  light 
between  crossed  nicols.  The  maximum  of  darkness  occurs  when  the 
line  of  intersection  on  the  mineral  plate  of  one  of  the  principal  sections 
bisecting  the  angle  between  the  optic  axes  becomes  parallel  to  one  of 
the  principal  sections  of  the  nicols.  Hence  the  sections  in  the  three 
principal  zones  extinguish  parallel  to  those  boundary  lines  or  cleavage 
cracks  which  run  parallel  to  a  crystallographic  axis.  The  maximum 


190          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

brightness  lies  at  45°  to  tins  position.  In  convergent"  light  sections 
perpendicular  to  a  bisectrix  exhibit  a  dark  cross  whose  arms  divide  the 
field  of  view  symmetrically  when  the  plane  of  the  optic  axes  coincides 
with  one  of  the  principal  sections  of  the  nicols.  Upon  a  rotation  of  the 
section  in  its  own  plane  the  cross  opens  to  hyperbolas  whose  poles  reach 
their  maximum  interval  after  a  rotation  of  45°.  This  interval  is  a 
measure  for  the  angle  between  the  optic  axes.  The  interference  figure 
is  bisymmetrical,  and  the  distribution  of  the  colors  shows  the  kind  of 
dispersion,  p  <  v  or  p  >  v.  How  much  of  the  interference  figure  is 
visible  depends  on  the  angle,  2Z?or  277,  according  to  whether  the  observa- 
tion is  made  in  air  or  in  oil.  In  sections  which  are  inclined  to  an  optic 
axis,  the  axial  bar  when  it  reaches  the  straight  position  passes  at  length 
through  the  locus  (point  of  egress)  of  the  second  axis.  If  the  locus  of 
the  bisectrix  of  an  interference  figure  is  not  in  the  centre  of  the  field 
of  view,  the  section  is  still  parallel  to  a  crystallographic  axis  when  one 
of  the  bars  bisects  the  field  in  the  crossed  position.  If  this  is  not  the 
•case  the  section  intersects  all  three  of  the  crystallographic  axes. 

In  general,  if  orthorhombic  minerals  exhibit  pleochroisni,  all  sec- 
tions are  dichroic  which  are  not  at  right  angles  to  an  axis.  The  greatest 
differences  of  color  lie  at  90°  to  one  another,  and  coincide  with  the 
positions  of  darkness  of  the  sections  between  crossed  nicols. 

Brookite. 
Literature. 

H.  THURACH,  Uber  das  Vorkommen  mikroskopischer  Zirkone  und  Titanmineralien 
in  den  Gesteinen.     Wiirzburg.  1884.  36-41. 

Brookite  appears  in  very  small  tabular  crystals  flattened  parallel  to 

oo  PQQ  (100),  which  are  bounded 
most  frequently  by  ooP  (110)  and 
P2  (122)  besides  oP  (001)  and 
2P&  (021),  more  rarely  also  by, 
mP£>  and  coPoo  (010).  The 
tabular  face  is  striated  parallel 
to  the  vertical  axis ;  the  crystals 
61  are  often  greatly  distorted,  and 

also  combined  to  form  twin-like 
or  irregular  groups.  Fig.  61  presents  some  small  brookite  crystals, 
according  to  Thiirach. 

The  cleavage  parallel  to  ooP<»  (010)  is  not  noticeable  on  the 
microscopic  crystals.  Brookite  is  transparent,  with  yellow  to  brown- 
ish-red color,  according  to  the  thickness  of  the  plates;  by  incident 


PSEUDOBROOKITE.  191 

light  it  exhibits  a  strong  adamantine  lustre,  somewhat  metallic,  and 
often  an  ashen-gray  color.  It  is  seldom  blue  or  greenish  blue.  The 
high  index  of  refraction  causes  strong  total  reflections  from  the  faces 
inclined  to  the  axis  of  the  microscope,  and  strong  relief  as  well  as 
a  rough  surface  in  Canada  balsam.  The  double  refraction  is  strong, 
the  interference  colors  therefore  are  high,  even  in  very  thin  plates. 
The  bisectrix  for  all  colors  stands  perpendicular  to  the  tabular  face, 
but  the  plane  of  the  optic  axes  for  red  and  yellow  light  is  parallel  to 
oP  (001),  while  for  more  strongly  refrangible  rays  it  is  parallel  to 
ooPoo  (010).  This  fact,  which  may  bo  easily  observed  in  convergent 
light  on  every  plate  by  employing  colored  glasses,  is  one  of  the  surest 
means  of  recognizing  this  mineral.  The  character  of  the  double 
refraction  is  positive,  consequently  a  =  c.  The  pleochroism  is  weak 
for  rays  vibrating  parallel  to  a  and  <?;  the  absorption  is  stronger  for 
the  first  than  for  the  second. 

The  sp.  gr.  =  3.8-4.15.  Chemical  composition,  TiO2 ;  reactions 
the  same  as  those  of  rutile  and  anatase. 

Brookite  possesses  no  characteristic  interpositions;  in  general  it 
appears  to  be  free  from  inclusions. 

Brookite  has  not  yet  been  found  in  fresh  eruptive  rocks  and  crystal- 
line schists.  Thurach  observed  it  especially  in  decomposed  granites, 
gneisses,  and  quartz  porphyries ;  it  frequently  occurs  together  with 
anatase.  He  also  discovered  it  in  many  sedimentary  rocks,  partly 
alone,  partly  in  company  with  anatase. 

Pseudobrookite. 

Literature. 

A.  KOCH,  Pseudobrookit,  ein  neues  Mineral.     T.  M.  P.  M.  1878.  I.  344-350. 

Pseudobrookite  forms  tabular  crystals,  mostly  with  rectangular  out- 
lines,whose  longest  dimension  is  less  than  2  mm., 
and  is  usually  below  1  mm.  The  boundary  ac- 
cording to  Koch  (Fig.  62)  is  generally  given  by 
a  =  oo  Poo  (100),  m  =  ooP  (110),  b  =  oopoo 
(010),  d  =  Poo  (101),  with  which  are  often  as- 
sociated  a  brachy prism  ooP2  (120)  and  a  derived 


macrodome  e  =  -JPS6  (103),   as  well   as  very 

small  pyramids.   The  most  important  angles  are 

a:m  =  154°  9',  and  a : d  =  138°  41'.  The  plates  d' 

are  often  only  partly  bounded  by  crystal  faces, 

and  are  always  vertically  striated  in  the  prism 

zone.      Cleavage   parallel   to  ooPoo  (010)  distinct;    Tornebohm   ob- 


a 


m 


192          PHYSIOGRAPHY  OF   THE  ROCK  MAKING  MINERALS. 

served  another  parallel  to  a  brachydome,  whose  traces  on  a  intersect 
one  another  at  60°. 

Crystals  of  some  thickness  are  opaque,  dark  brown  to  black,  and 
possess  a  metallic  adamantine  lustre;  very  thin  plates  are  transparent, 
brownish,  or  ruby  red,  strongly  refracting  with  moderate  double  refrac- 
tion. Nothing  is  known  concerning  the  position  and  angle  of  the 
optic  axes.  The  pleochroisrn  is  weak ;  the  ray  vibrating  parallel  to  c 
is  most  strongly  absorbed. 

Sp.  gr.  =  4.98.  The  chemical  composition  is  not  known  exactly ; 
an  incomplete  analysis  gave  Koch  52.74  TiO2,  42.29  Fe2O3  with  some 
A12O3 ;  the  residue  was  lime  and  magnesia.  The  powder  is  soluble  in 
concentrated  hydrochloric  or  sulphuric  acid  after  long  heating.  The 
mineral  is  difficultly  fusible,  yields  an  iron  bead  with  borax,  and  a 
titanium  bead  with  salts  of  phosphorus. 

Pseudobrookite  has  been  found  in  cracks  and  hollows  in  the 
andesite  of  Aranyer  Berg  in  Siebenbiirgen,  Transylvania,  in  the  apatite 
of  Jumilla,  Spain,  in  the  domite-like  trachytes  of  Fayal  and  San 
Miguel,  in  an  augite  andesite  of  Behring's  Island,  in  the  basalt  debris 
of  Kreuzberg,  in  a  basalt  tuff  and  in  the  phonolite  debris  of  Kiiuling. 
It  is  very  abundant  in  a  Central  American  amphibole  andesite  of  Mira- 
valles,  Costa  Rica. 

Aragonite. 

Aragonite  never  forms  crystals  in  rocks,  but  masses  or  prismatic 
aggregates  with  parallel,  divergent,  or  radial  arrangement  of  the  indi- 
viduals. 

The  cleavage  parallel  to  the  brachipinacoid  is  not  at  all  or  but 
slightly  noticeable. 

Aragonite  is  transparent  and  colorless;  the  double  refraction  is 
very  strong,  ana  —  1.5301,  f3na  =  1.6816,  yna  =  1.6859.  The  optic  axes- 
lie  in  the  rnacropinacoid,  and  c  is  the  negative  bisectrix,  2  V=  18°. 
Prisms  cut  at  right  angles  to  their  axis  give  the  interference  figure  of 
an  orthorhombic  crystal  with  p  <  v .  Pleochroisrn  not  noticeable. 

Sp.  gr.  =:  2.94,  Chemical  composition  =  CaO,  CO2.  Reactions  the 
same  as  for  calcite,  from  which  it  is  easily  distinguished  by  the  absence 
of  cleavage  and  the  specific  gravity.  It  occurs  as  a  decomposition 
product  in  the  basic  eruptive  rocks. 

Anhydrite. 

Anhydrite  forms  granular  or  columnar  fibrous  aggregates.  The 
grains  appear  to  be  quite  regularly  developed  in  three  directions.  In 


ANDALUSITE.  193 

them  twin  lamellae  often  lie  diagonal  to  two  cleavages,  and  are  pres- 
sure phenomena. 

Cleavage  parallel  to  three  pinacoids  is  distinctly  shown  microscopi- 
cally by  cracks  of  different  degrees  of  sharpness  and  of  varying  fre- 
quency. Most  perfect  cleavage  parallel  to  oP  (001),  nearly  as  perfect 
parallel  to  ooPoo  (010),  less  perfect  parallel  to  ooPoo  (100). 

In  thin  section  transparent  and  colorless.  Miller  determined  oc  = 
1.571,  ft  =  1.576,  y  =  1.614.  The  axial  plane  lies  in  ooPoo  ;  the  acute 
bisectrix  coincides  with  a.  The  character  of  the  double  refraction  is 
positive.  Cleavage  plates  parallel  to  the  most  imperfect  cleavage 
exhibit  the  axial  figure  very  finely.  ^E—  71°  10'-71°  20'. 

Sp.  gr.  =  2.8-3.  Chemical  composition  =  CaO,  SO3.  But  slightly 
attacked  by  acids ;  together  with  fluorite  it  melts  to  a  clear  bead. 
Anhydrite  changes  into  gypsum  by  taking  up  water  under  the  action 
of  the  atmosphere.  It  forms  anhydrite  rock,  which  has  been  investi- 
gated microscopically  by  Fr.  Hammerschmidt  (T.  M.  P.  M.  1882,  Y. 
245).  It  also  occurs  in  gypsum. 

Andalusite. 
Literature. 

H.  ROSENBUSCH,  Die  Steiger  Schiefer  und  ihre  Contactzone  an  den  Granititen  von 
Barr-Andlau  und  Hohwald.  Strassburg  i.  E.  1877.  (cf.  also  N.  J.  B.  1875.  849.) 

Andalusite  never  occurs  massive,  but  always  in  more  or  less  well- 
defined  crystals;  seldom  in  rounded  grains.  The  dimensions  vary 
from  a  length  of  several  centimetres  to  hundredths  of  millimetres, 
but  the  relation  of  length  to  breadth  is  quite  constant — about  1:3  to 
1 :4.  The  forms  of  imbedded  crystals  are  very  simple,  ooP(HO)  with 
an  angle  of  90°  50',  and  oP  (001),  to  which  a  dome  is  occasionally 
added.  Hence  cross-sections  are  very  nearly  quadratic,  longitudinal 
sections  elongated  rectangles  (PI.  XVII.  Fig.  2).  Twinning  does  not 
seem  to  occur. 

The  quite  perfect  cleavage  parallel  to  ooP  (110)  shows  itself  in  the 
cross-sections  of  the  larger  individuals,  generally  in  very  distinct 
cracks,  which  cut  each  other  apparently  at  right  angles  and  lie  parallel 
to  the  crystallographic  boundaries ;  in  longitudinal  sections  they  run 
parallel  to  each  other  and  to  the  boundaries  of  the  crystal.  The  cleav- 
ages parallel  to  the  dome  and  to  the  vertical  pinacoid  are  seldom 
noticeable.  In  the  very  small  microscopic  individuals  the  cleavages 
are  frequently  not  noticeable  at  all.  In  the  larger  individuals  of 
andalusite  and  chiastolite  one  may  often  observe  that  an  apparent 
twinning  line  passes  diagonally  through  the  cross-section.  This  is  due 
13 


194 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


to  mechanical  deformation,  the  face  ooPoo  (100),  and  never  the  other 
pinacoid,  having  acted  as  a  gliding  plane,  and  the  halves  of  the  crystal 
have  been  pushed  out  of  an  exact  parallel  position  by  an  extremely 
thin  wedge  of  the  rock  mass. 

Andalusite  is  usually  colorless,  seldom  transparent  reddish.  The 
index  of  refraction  is  quite  high,  hence  the  distinct  relief ;  the  double 
refraction  is  weak,  and  the  interference  colors  consequently  low.  The 
thin  section  must  be  at  least  0.05  mm.  thick  to  give  red  of  the  1st 

order  under  the  most  favorable  circum- 
stances. This  is  an  important  means  of 
distinction  from  sillimanite.  The  charac- 
ter of  the  double  refraction  is  negative. 
The  plane  of  the  optic  axes  lies  in  ccPZo 
(010) ;  the  vertical  axis  is  the  bisectrix ; 
thus,  &=c,  &=fc,  c=&.  Fig.  63  gives  the 
5=5  optical  scheme  in  erystallographic  orienta- 
tion. 2  V=  83°-85°;  a  =  1.632,  ft  =  1.638, 
y=  1.643.  Cross-sections  show  the  point 
of  egress  of  a  bisectrix,  sections  parallel  to 
cePab  (010)  give  the  highest  interference 
colors.  The  extinction  lies  diagonally  in 
cross-sections,  and  parallel  to  the  cleavage 
A  and  crystal  boundaries  in  longitudinal  sec- 

ing.  63  tions.    In  unsymmetrical  sections  a  consid- 

erable extinction  angle  may  show  itself  because  of  the  large  optic  angle. 
Pleochroism  is  sometimes  completely  wanting,  while  in  other  cases  it  is 
very  strong ;  in  rather  thick  sections  d  =  olive-green,  b  =  oil-green,  c  — 
dark  blood-red ;  in  thin  sections  #  =  £=  quite  colorless  to  very  pale 
greenish,  <?  —  rose-red.  The  absorption  is  c  >  5  >  a.  Sections 
parallel  to  a  dome  and  perpendicular  to  an  optic  axis  exhibit  distinctly 
the  polarization  brush. 

Sp.  gr.  =  3.16-3.20.  Chemical  composition  =  A12O3,  SiO2.  An- 
dalusite is  not  attacked  by  acids,  even  by  hydrofluoric,  and  is  therefore 
easily  isolated  chemically ;  its  isolation  by  means  of  separating  solu- 
tions also  is  not  difficult.  The  isolated  powder  when  heated  to  redness 
with  cobalt  solution  is  colored  a  fine  blue.  Andalusite  is  readily  altered 
through  the  action  of  the  atmosphere  into  laminated  and  fibrous  aggre- 
gates, which  may  belong  in  part  to  muscovite  (sericite),  in  part  to  kaolin. 
Andalusite  possesses  no  constant  microstructure :  sometimes  it  en- 
closes the  minerals  associated  with  it,  such  as  quartz,  biotite,  and  the 
ores,  as  well  as  fluid  inclusions;  very  frequently  carbonaceous  particles, 
about  which  pleochroic  halos  often  appear.  These  are  yellow  when 


SILLIMANITE.  195 

the  vertical  axis  stands  parallel  to  the  principal  section  of  the  nicol, 
and  disappear  upon  a  rotation  of  90°.  Glowing  destroys  them ;  they 
may  therefore  be  due  to  finely  divided  organic  pigments,  which  also 
appear  to  produce  the  pleochroism  of  the  andalusite. 

Andalusite  is  highly  characteristic  of  metamorphic  schists  ;  its  true 
home  is  the  contact  zones  of  clay  slates  near  granites,  syenites,  elaeolite 
syenites,  and  diorites.  It  occurs  far  more  rarely  in  the  mica  schists 
and  gneisses  of  the  Archaean,  where  also  it  is  probably  of  metamorphic 
origin.  According  to  E.  Cohen  (N.  J.  B.  1887.  B.  II.)  andalnsite  is 
not  an  uncommon  accessory  constituent  of  normal  granites,  where  it 
occurs  in  the  form  of  columns,  either  acicular  or  with  a  more  compact 
habit,  which  are  always  isolated  and  riot  grouped  like  those  in  contact 
zones  and  in  the  crystalline  schists.  The  terminations  are  often 
incomplete,  rounded  or  jagged ;  sometimes  they  are  terminated  by  two 
inclined  lines,  probably  derived  from  a  dome  or  pyramid. . 

Chiastolite  (PL  XVII.  Fig.  3)  is  chemically,  crystallographically, 
and  physically  identical  with  andalusite,  and  is  only  distinguished  from 
it  by  the  constancy  with  which  carbonaceous  substances  are  enclosed  in 
it,  arranged  in  the  well-known  manner. 

Andalusite  may  be  confused  with  diopside  upon  hasty  observation 
on  account  of  the  similar  cleavage,  with  sillimanite  (in  granulite)  be- 
cause of  the  same  chemical  composition,  with  zoisite  and  feldspar  from 
a  certain  similarity  in  habit  and  in  the  decomposition  products.  Diop- 
side is  easily  distinguished  by  its  much  stronger  double  refraction  and 
the  monoclinic  behavior  of  all  sections  not  lying  in  the  orthodiagonal 
zone.  Andalusite  may  be  distinguished  from  sillimanite  by  determin- 
ing the  value  of  the  vertical  axis  of  elasticity,  from  zoisite  by  the  cleav- 
age arid  chemical  reaction,  from  feldspar  by  the  noticeably  higher  index 
of  refraction,  as  well  as  by  the  chemical  reaction  and  specific  gravity. 


/Sillimanite. 
Literature. 

E.  KALKOWSKY,  Die  Gneissformation  des  Eulengebirges.     Leipzig.  1878.  p.  5  sqq. 
A.  MICHEL-LEVY,  Sillimanite  dans  le  gneiss  du  Morvan.     Bull.  Soc.  min.  Fr.  1880. 

III.  30. 

/Sillimanite  as  a  constituent  of  rocks  always  forms  long  prismatic 
-crystals,  which  are  very  thin,  and  are  only  recognizable  macroscopically 
when  they  are  grouped  in  felt-like  aggregates.  The  dimensions  of  the 
crystals  vary  greatly,  but  the  length  is  always  greatly  in  excess  of  the 
breadth ;  they  sink  to  such  fine  needles  that  they  are  scarcely  trans- 
parent even  with  the  strongest  magnifying  powers.  In  the  prism  zone 


196         PHYSIOGRAPHY  OF   THE  ROCK-MAKING  MINERALS. 

the  boundary  is  given  by  crystal  faces,  either  by  the  fundamental  prism 
with  the  angle  110  :  110  =  111°,  or  by  a  combination  of  this  with  the 
prism  ooPf  (230)  with  an  anterior  angle  of  88°-89°,  or  by  the  latter- 
alone.  The  pinacoidal  faces  ooPco  (010)  and  ooPoo  (100)  are  com- 
paratively rare.  Terminal  faces  are  not  definitely  recognizable ;  the 
crystals  apparently  break  off  or  are  very  finely  pointed.  The  cross-sec- 
tions are  rhombic,  octagonal,  or  apparently  quadratic,  and  then  suggest 
andalusite  very  strongly;  the  longitudinal  sections  are  long  lath-shaped. 
The  individuals,  however,  are  usually  so  thin  that  they  lie  in  the  thin 
sections  as  whole  bodies  (PI.  XVII.  Fig.  4).  Their  surface  is  often 
striated  parallel  to  the  vertical  axis,  and  their  cross-sections  rounded 
and  notched. 

The  cleavage  parallel  to  the  macropinacoid  shows  itself  in  very  fine 
parallel  cracks  in  both  longitudinal  and  cross  sections  of  the  larger 
individuals,  but  is  not  noticeable  in  the  very  microscopic  individuals. 
All  individuals  which  are  not  too  short  exhibit  a  transverse  parting,, 
the  segments  being  sometimes  separated  by  the  rock  mass  (mostly 
quartz).  Sillimanite  needles  never  occur  bent  or  curved,  but  are 
frequently  broken.  H.  =  6-7. 

SiHimanite  in  thin  section  is  transparent  and  colorless ;  the  index 
of  refraction  is  somewhat  higher  than  for  andalusite,  fip  —  1.660, 
according  to  Des  Cloizeaux;  the  double  refraction  is  considerable^ 
y  —  a  =  0.020-0.022  according  to  Michel-Levy;  and  the  interference 
colors  in  thin  section  are  higher,  from  the  upper  half  of  the  1st  order 
and  the  lower  half  of  the  2d  order.  The  plane  of  the  optic  axes  lies 
in  the  macropinacoid,  the  vertical  axis  is  the  positive  bisectrix,  thus 
c  =  c,  which  is  a  certain  and  convenient  means  of  distinguishing  it 
from  andalusite.  The  angle  of  the  optic  axes  is  small,  %E  =  44°. 
Cross-sections  in  thin  section  exhibit  a  distinct  axial  figure.  Sillimanite 
from  Saybrook,  Conn.,  is  strongly  pleochroic;  cleavage  plates  paral- 
lel to  ooPoo  (100)  give  for  c  dark  clove-brown,  for  b  light  brownish ; 
rock-making  sillimanite  is  not  noticeably  pleochroic  in  thin  section. 

Sp.  gr.,  =  3.23-3.24,  is  higher  than  for  andalusite.'  Chemical  com- 
position and  reaction  are  the  same  as  for  andalusite.  Sillimanite  is 
usually  perfectly  free  from  inclusions.  Decompositions  lead  to  kaolin. 

Sillimanite  is  one  of  the  most  characteristic  minerals  of  the  crystal- 
line schists,  especially  of  the  feldspathic  gneisses,  in  which  it  is  some- 
times distributed  generally  through  all  the  constituents,  with  the 
exception  of  the  feldspars,  at  others  it  is  intimately  combined  with 
fibrous  quartz  in  the  form  of  lenticular  knots,  called  fibrolite, 
bucholzite,  wdrthite,  monrolite,  xenolite.  It  is  frequently  accompanied 


TOPAZ.  197 

by  cordierite  in  cordierite  gneisses  and  kinzigites.     It  is  also  found  to 
some  extent  in  rocks  exhibiting  contact  metamorpliism. 

Silliraanite  may  be  confounded  microscopically  with  andalusite  and 
zoisite.  For  its  distinction  from  the  first,  see  Andalusite.  From  zoisite 
it  is  distinguished  by  its  strong  double  refraction  and  its  chemical 
resistance. 

'  V  Topaz. 

As  a  rock  constituent  topaz  has  been  observed  almost  always  in 
•crystals,  very  rarely  in  irregular  grains  or  masses ;  the  crystals  have  a 
short  prismatic  habit,  and  are  bounded  principally  by  the  faces  ooP(llO) 
and  2Po6  (021)  with  92°  42',  or  4Po6  (041)  with  55°  20'.  In  addition 
the  pyramids  and  the  prism  oojP2  (120)  are  generally  very  subordinate. 
The  base  is  usually  wanting,  or  is  very  slightly  developed.  The  crystals, 
sometimes  blue,  sometimes  colorless  or  light  yellow,  seldom  attain  macro- 
scopic dimensions,  and  are  usually  first  recognized  microscopically. 

The  perfect  cleavage  parallel  to  oP  (001)  shows  itself  by  distinct 
parallel  cracks  in  all  sections  which  are  not  parallel  to  the  base.  H.  =  8. 

Topaz  is  always  perfectly  transparent  in  thin  section  ;  the  index  of 
refraction  in  consequence  of  its  fluorine  percentage  is  lower  than  is  to 
be  expected  for  a  gem  ;  the  double  refraction  is  weak,  about  the  same 
as  that  of  quartz.  Hence  the  relief  is  not  strong,  the  interference 
colors  quite  low;  in  good  thin  sections  they  scarcely  exceed  yellow 
of  the  1st  order.  On  colorless  Brazilian  topaz  Hud  berg  determined 
<xna  =  1.612,  PM  =  1.614,  yna  =  1.621 ;  on  Schneckenstein  topaz,  Des 
Oloizeaux  found  <xp  =  1.614,  ftf  =  1.616,  yp  =  1.623. 

The  axial  plane  lies  in  the  brachypinacoid,  and  the  vertical  axis  is 
the  acute  bisectrix.  The  character  of  the  double  refraction  is  positive: 
thus,  G  —  C,  a  =  a,  I  =  b.  The  axial  angle  varies  between  wide  limits, 
even  in  plates  from  the  same  crystal — according  to  Des  Cloizeaux  from 
about  70°  to  120°  in  air.  The  dispersion  is  quite  strong,  p  >  v.  In 
thin  sections  the  interference  figure  of  both  axes  is  obtained  on  those 
sections  which  show  no  cleavage — an  important  criterion  for  diagnosis. 
Optical  anomalies  are  frequent,  especially  in  the  yellow  Brazilian  topaz, 
where  they  were  first  observed  by  Brewster  ;*  but  in  rock-making  topaz 
they  are  weaker  and  less  frequent.  Pleochroism  is  not  noticeable  in 
thin  section. 

Sp.  gr.  =  3.52-3.56.  Chemical  composition  =  5AlaO3,  SiO,,  + 
A12F16,  SiFl4.  Acids  have  no  action  upon  topaz.  Through  decomposi- 
tion topaz  loses  its  fluorine,  and  by  taking  up  water  passes  into  kaolin 

*  Trans.  Cambridge  Phil.  Soc.  1822. 


198 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


(nakrite),  or  by  the  addition  of  water  and  alkali  passes  into  muscovite." 
The  latter  process  has  been  studied  by  J.  S.  Diller  and  F.  "W.  Clarke* 
in  the  topaz  of  Stoneham,  Me.;  the  formation  of  mica  advances  along 
the  cleavage  planes  and  other  cracks. 

Topaz  usually  encloses  besides  plates  of  hematite  and  ilmenite 
abundant  fluid  inclusions,  which  sometimes  lie  in  lines  and  rows,  and 
at  others  are  arranged  approximately  along  concentric  faces  parallel  to- 

coP  (110)  and  ooP2  (120), 
Their  form  is  very  vari- 
able and  striking:  they 
sometimes  consist  of  water 
and  aqueous  solutions  £ 
sometimes  of  liquid  carbon 

]   \  ^/V-^  /  dioxide,  or  of  both  together. 

In  the  fluid  inclusions  of 
many  topazes  are  crystal- 
line secretions  (Fig.  64)r 
among  which  colorless  cubi- 
cal  crystals  are  the  most  fre- 
quent. These  dissolve  in 

their  mother-liquor  when  sufficiently  heated,  and  crystallize  out  again 
upon  cooling.  Hence  they  can  scarcely  be  referred  to  rock-salt.  Les& 
frequently  there  are  rhombohedral  colorless  crystals,  long  needle- 
shaped  microlites  generally  crossing  one  another  as  if  twinned,  and 
reddish-brown  pyramidal  crystals  with  a  truncated  point ;  these  crystal- 
lizations are  not  dissolved  in  the  fluid  upon  heating.  Topaz  is  peculiar 
to  all  granitic  rocks  which  carry  tin  ore,  and  is  particularly  constant  in 
the  greisen.  It  is  also  sparingly  met  with  in  granitic  rocks,  especially 
when  they  bear  fluorite  or  tourmaline,  as,  for  example,  many  Cornwall 
occurrences. 

Staurolite. 
Literature. 

A.  VON  LASAULX,  Ueber  Staurolith.     T.  M.  M.  1872.  II.  173. 
K.  PETERS  und  R.  MALY,  Ueber  den  Staurolith  von  St.  Radegund.  S.  W.  A.  1868. 
LVII.  646. 

Staurolite  always  appears  as  single  individuals  or  twins,  which 
occasionally  assume  the  form  of  grains  through  the  imperfect  develop- 
ment of  the  crystal  faces.  The  absence  of  elongated  forms  is  charac- 
teristic. The  forms  (Fig.  65)  are  very  constant,  m  =  ooP  (110)  with 


*  Amer.  Journ.  1885.  XXIX.  May.  378-384. 


8TA  UROLITE. 


199 


129°  20',  p  =  oP  (001),  r  =  Poo  (101),  mostly  very  small,  often  want- 
ing, o  =  ooPob  (010),  often  very  small,  and  at  times  absent.  Hence 
the  cross-sections  are  acutely  rhombic  or  almost  hexagonal,  the 
longitudinal  sections  broad  rectangles  (parallel  to  ooPoo)  or  narrow 
ones  (parallel  to  00  P  06  ).  The  twinning  parallel  to 
f  Poo  (032)  and  fPf  (232)  is  the  same  for  micro- 
scopic  crystals  as  it  is  for  the  large  individuals. 
Often  the  twinning  is  not  noticeable  in  the  outline 
of  the  crystal,  a.s  one  individual  may  be  wholly 
enclosed  in  the  other,  but  is  recognized  optically 
by  the  position  of  the  axial  planes  or  by  the  pleo- 
chroisrn.  A  laminated  structure  occurs  parallel 
to  oP,  producing  a  parting  parallel  to  this  face 
which  resembles  a  cleavage. 

The  cleavage  parallel  to  ooPoo  (010)  and  ooP  (110)  is  variable,  show- 
ing itself  at  times  in  sharp  cracks,  especially  in  the  short  diagonal  of 
the  cross-section,  and  by  parallel  cracks  in  the  longitudinal  sections ; 
at  times  it  is  scarcely  noticeable.  H.  =  7—7.5. 

Staurolite  becomes  transparent  and  yellowish  to  reddish  brown 
according  to  the  thickness  of  the  section  and  its  position.  The  index 
of  refraction  is  very  high — according  to  Miller  fip  =  1.7526 ;  Des 
Cloizeaux,  ftp  =  1.749  ;  therefore  the  marginal  total  reflection  is  very 
strong,  the  relief  considerable,  the  surface  very  rough.  The  double 
refraction  is  strong;  the  interference  colors  are  brilliant  even  in  very 
thin  sections.  The  axial  plane  lies  in  the  macropinacoid,  the  angle  for 
red  rays  is  about  89°,  the  character  is  positive,  the  vertical  axis  is  the 
first  bisectrix ;  the  optical  scheme  is  indicated  in  Fig.  65.  Cross-sec- 
tions, even  in  quite  inclined  positions,  give  interference  ligures  in  con- 
vergent light,  which  show  that  the  axial  plane  lies  in  the  longer  diago- 
nal of  the  prismatic  cleavage ;  this  is  the  best  means  of  distinguishing 
it  from  titanite,  which  often  resembles  it  closely.  The  dispersion  is 
weak,  p  >  v.  The  pleochroism  is  distinct,  though  not  strong :  c  hya- 
cinth-red to  blood-red ;  a  and  b  yellowish  red,  often  with  a  tinge  of 
green.  This  pleochroism  is  often  noticeably  stronger  around  inter- 
positions than  in  the  main  mass  of  the  mineral,  although  there  is  no 
difference  in  the  intensity  of  the  coloring  in  ordinary  light. 

Sp.  gr.  =  3.4-3.8,  varying  greatly  with  the  amount  of  the  many 
kinds  of  interpositions;  it  is  higher  as  the  mineral  is  purer.  Chemical 
composition  not  known  with'  certainty,  approximately  represented  by 
the  formula  FeO,  2A12O3,  2SiO2,  in  which  a  small  part  of  the  FeO  is 
replaced  by  MgO.  Is  not  acted  on  by  acids  including  hydrofluoric. 


200         PHYSIOGRAPHY   OF  THE  ROCK-MAKING  MINERALS. 

It  is  therefore  easily  separated  from  the  rocks  by  chemical  methods, 
and  from  the  isolated  minerals  accompanying  it  by  means  of  its  spe- 
cific gravity  and  by  an  electro-magnet. 

The  larger  crystals  of  staurolite  are  made  very  impure  by  inclusions 
of  the  minerals  associated  with  them  (tourmaline,  rutile,  mica,  disthene, 
etc.),  but  they  are  especially  impregnated  with  quartz  grains  carrying 
rings  of  dark  interpositions  of  carbonaceous  matter.  The  microscopic 
individuals,  on  the  other  hand,  are  usually  much  purer  and  often  com- 
pletely free  from  admixtures.  Decomposition  products  are  rare,  and 
generally  occur  only  along  the  cracks  in  the  larger  crystals;  they  appear 
to  be  chlorite  and  a  green  mica. 

Staurolite  does  not  occur  in  eruptive  rocks;  it  is  particularly  a 
mineral  of  the  Archaean  rocks,  and  is  very  frequently  accompanied  by 
disthene.  It  is  very  common  in  gneiss  and  mica  schists,  but  does  not 
occur  in  schists  rich  in  amphibole. 

The  Group  of  Orthorhoml>ic  Pyroxenes. 

Literature. 

F.  BECKE,  Ueber  die  Unterscheidung  von  Augit  und  Bronzit  in  Diinnschliffen.  T. 

M.  P.  M.  1883.  V.  527. 
J.  BLAAS,  Petrographische  Studien  an  jungeren  Eruptivgesteinen  Persiens.  T.  M, 

P.  M.  1880.  III.  479  sqq. 

H.  BUCKING,  Bronzit  vom  Ultenthal,  Z.  X.  1883.  VII.  502. 
F.  FOUQUE,  Sur  1'hypersthenc  de  la  ponce  de  Santorin.  Bull.  Soc.  min.  Fr.  1878. 

III.  46. 

J.  A.  KRENNER,  Ueber  den  Szaboit.     Z.  X.  1884.  IX.  255. 
H.  ROSENBUSCH,  Die  Gesteinsarten  von  Ekersund.  Nyt  Mag.  for  Naturvid.  XX VII. 

1883. 

Ueber  den  Sagvandit.  N.  J.  B.  1884.  I.  195. 

F.  SVENONIUS,  Bronzit  fran  Frostvikens  socken  i  Jamtland.  Geol.  FOr.  i  Stockholm 

F5rhdlg.  1883.  VI.  204. 

G.  TSCHERMAK,  Mikroskopische  Unterscheidung  der  Mineralien  der  Augit-,  Amphi- 

bol-  und  Biotitgruppe.  S.  W.  A.  1869.  LIX.  1.  Abthl. 
—    Ueber  Pyroxen  und  Amphibol.  T.  M.  M.  1871.  I.  17—21. 
B.  WEIGAND,  Die  Serpentine  der  Vogesen.  T.  M.  M.  1875.  183. 

The  orthorhombic  pyroxenes  occur  in  two  ways  in  rocks ;  they 
either  form  short  prismatic  crystals,  perfectly  developed,  of  small  or 
microscopic  dimensions  in  certain  porphyritic  eruptive  rocks;  or  they 
appear  in  lamellar  crystalloids  and  aggregates,  often  of  very  consider- 
able dimensions,  seldom  microscopic,  which  occur  in  certain  granular 
eruptive  rocks  of  the  oldest  geological  epochs,  as  well  as  in  many 
members  of  the  crystalline  schists.  The  prismatic  crystals  (Figs.  66 


ORTHORHOMBIC  PYROXENES. 


201 


.and  67)  when  referred  to  the  axes  d\~b\c  —  0.9T133 : 1 : 0.57000  (that 
is,  with  the  obtuse  prism  angle  in  front)  show  a  =  QO  JP  06  (100),  J  = 
ooPoQ  (010)  predominant,  and  m  =  ooP  (110)  subordinate ;  also  e  = 
P2  (212),  i  =  2P2  (211),  o  =  P  (111),  k  =  iPoo  (012).  The  value 
of  the  prism  angle  is  about  92°.  In  sections  parallel  to  ooP^o  (100)  the 
terminal  edges  of  P2  (212)  intersect  at  148°  IV,  those  of  2P2  (211) 
at  120°  38';  in  sections  parallel  to  ooPdo  (010)  these  terminal  edges 
intersect  at  119°  11'  and  80°  52'  respectively.  Cross-sections  through 
the  crystals  present  rectangles  with  truncated  corners.  Sections 
from  the  vertical  zone  present  long  strips  pointed  at  both  ends.  Sec- 
tions of  irregular  masses  naturally  exhibit  no  regular  outline  and  must 
be  oriented  by  the  cleavage.  Twinning  is  quite  rare;  the  pyroxene 
-crystals  in  the  porphyrites  and  andesites  are  sometimes  intergrown  in 
such  a  manner  that  the  individuals  have  the  faces  ooPoo  (010)  in  com- 
mon, and  appear  twinned  after  a  macrodome.  From  the  inclination  of 


m     a 


m     a 


m 


"Fig.  GG 


Fig.  67 


the  vertical  axis,  POO  (101)  may  be  considered  as  the  probable  twin- 
ning plane  ;  but  there  appear  to  be  other  faces  in  the  same  zone  which 
occasionally  act  as  twinning  planes.  In  massive  bronzite  of  the  nor- 
ites,  peridotites,  and  crystalline  schists  a  twinning  parallel  to  JPoo  (014) 
is  not  at  all  uncommon.  This  last  twinning,  however,  appears  to  be 
secondary,  arising  from  mechanical  causes,  as  is  indicated  by  the 
'"jogging"  phenomena  in  the  twinned  individuals  in  the  immediate 
neighborhood  of  the  composition  plane. 

The  cleavage  in  all  orthorhombic  pyroxenes  lies  parallel  to  the 
prism  of  about  92° ;  the  perfection  of  this  cleavage  varies,  but  it  is 
always  noticeable  in  convergent  light  upon  careful  observation.  In 
cross-sections  of  the  crystals  the  cracks  corresponding  to  this  cleavage 


202          PHYSIOGRAPHY  OF  THE  ROCK-MAKING   MINERALS. 

run  parallel  to  the  small  faces  which  truncate  the  pinacoidal  edges.  ID 
the  massive  varieties  in  the  older  rocks,  besides  the  prismatic  cleavage 
there  is  always  a  more  perfect  one  parallel  to  oojPoo'  (010) ;  they  also 
show  cracks  which  indicate  an  imperfect  parting  parallel  to  00^55  (100). 
This  last  cleavage  is  seldom  found  in  the  crystals  occurring  in  the  por- 
phyritic  rocks  and  lavas.  Cross-sections  of  the  massive  forms,  therefore, 
are  traversed  by  a  double  system  of  cleavage  cracks  apparently  intersect- 
ing at  right  angles,  and  bisecting  each  other's  angles  (PI.  X.  Fig.  2). 
In  sections  from  the  prism  zone  all  the  cleavage  cracks  run  parallel  to 
the  vertical  axis.  It  is  not  unlikely  that  the  pinacoidal  partings  in 
some  cases  correspond  to  gliding  planes,  and  are  the  result  of  mountain 
pressure,  while  in  others  they  are  brought  about  by  the  inclusion  of 
foreign  substances.  Furthermore,  an  irregular  cracking,  approximately 
perpendicular  to  the  vertical  axis,  is  observed  both  in  the  crystals  and 
lamellar  masses,  which,  in  spite  of  its  general  distribution,  does  not 
correspond  to  a  cleavage.  The  latter  cracks  play  a  great  role  in  the 
decomposition  of  these  minerals. 

The  orthorhombic  pyroxenes  become  transparent  in  various  colors, 
according  to  the  position  of  the  section  and  to  the  iron  percentage. 
Enstatite  is  almost  colorless  to  grayish  or  yellowish  white;  bronzite  is 
yellowish  to  greenish ;  hypers  then  e  green,  light  red,  or  brownish  red. 
The  index  of  refraction  is  high,  and  appears  to  increase  with  the  iron 
percentage;  hence  the  marginal  total  reflection  is  strong,  the  surface 
distinctly  rough. 

Bronzite  from  Kupferberg,  J3  ==  1.668  (Des  Cloizeaux). 

Hypersthene  fi'om  Lauterbach,  fi  =  1.685  (Des  Cloizeaux). 

Hypersthene  from  Soggendal,  ft  =  1.7125  (Sanger). 

Hypersthene  from  St.  Panl'a  Island,       y  —  1.7270  (J.  E.  Wolff) ; 

<*  =  1.7158  (J.  E.  Wolff). 

The  double  refraction  is  weak  for  members  of  the  series  poor  in 
iron:  Michel-Levy  determined  on  bronzitc  from  Lherz,  Pyrenees,. 
y  —  a  =  0.010  ;  on  pale  brown  hypersthene  from  Arvien,  y  —  a  = 
0.0115  ;  from  the  figures  given  above  for  hypersthene  from  St.  Paul's 
Island,  v  —  a  =  0.0112.  Hence  the  interference  colors  are  low  for 
enstatite  and  bronzite,  not  exceeding  yellow  of  the  1st  order;  for 
hypersthene  they  are  noticeably  higher,  reaching  red  of  the  1st  order 
in  sections  which  are  not  too  thin. 

The  direction  of  extinction  in  the  principal  zones  lies  parallel  to  the 
pinacoidal  cleavages  and  diagonal  to  the  prismatic.  The  vertical  axis 
is  the  axis  of  least  elasticity,  the  brachydiagonal  is  that  of  greatest 


ORTHORIIOMBIG  PYROXENES. 


203 


elasticity ;  thus,  c  =  C,  a  =  a,  b  =  fc.  Consequently  the  plane  of  the- 
optic  axis  always  lies  in  the  brachypinacoid.  The  angle  between  the 
optic  axes  varies  considerably,  chiefly  with  the  iron  percentage :  for 
enstatites  and  bronzites  the  vertical  axis  is  the  acute  bisectrix — they 
are  optically  positive  ;  for  hypersthenes  a  is  the  acute  bisectrix — they 
are  negative.  On  account  of  the  high  index  of  refraction  the  observa- 
tions must  be  made  in  oil.  The  following  table  shows  clearly  the  de- 
crease in  the  negative  axial  angle  with  increasing  iron  percentage. 


In  Oil. 
133° 


FeO+MnO.  Locality. 

......  2.76$ Enstatite Mahren Des  Cloizeaux. 

123°  38' 5.77    Bronzite Leiperville. ... 

114°  15' 11.14    "       Greenland.... 

112°  30' 8.42   "       Balsf  jord Kosenbuscn. 

106°  51' 9.86    ' '       Kraubat Tschennak. 

101°  30' 10.62   "       Lauterbach...  Des  Cloizeaux. 

98°       13.58    Meteorite Breiterbach. . .  v.  Lang. 

98°  22' 15.14   Hypersthene Farsund Des  Cloizeaux. 

85°  39'..  ..22.59.  "  ..Labrador... 


59°  20' 33.6 


Mont  Dore — Krenner. 


The  dispersion  about  the  negative  bisectrix  is  quite  weak  for  those 
members  of  the  series  poor  in  iron  ;  stronger,  p>  v,  for  those  rich  in 
iron. 

The  pleochroism  changes  with  the  iron  percentage.     It  is  scarcely 


P 


A 

-,1111 


-.  68 


Fig.  G9 


or  not  at  all  noticeable  in  the  enstatites  and  bronzites  poor  in  iron  ;  in 
the  more  ferruginous  bronzites  the  color  of  the  rays  vibrating  parallel  to 
c  is  pale  grayish  green  ;  those  parallel  to  a  and  5  are  quite  uniformly 
pale  yellow  to  pale  grayish  yellow.  For  the  hypersthenes  a  is  red- 
dish brown,  b  is  reddish  yellow,  c  is  green  ;  the  absorption  is  slightly 
pronounced.  The  pleochroism  diminishes  rapidly  as  the  thickness  of 
the  section  decreases.  The  axial  plane  always  lies  in  the  plane  of  the 


204         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

green  and  brownish-red  rays  ;  hence  the  interference  figure  is  found  IK. 
the  less  pleochroic  sections,  and  the  axial  bar  then  lies  parallel  to  the 
perfect  pinacoidal  cleavage.  Figs.  68  and  69  present  graphically  the 
*  phenomena  of  cleavage  and  optical  orientation  just  described.  As  a 
means  of  distinction  from  the  monoclinic  pyroxenes  it  is  to  be  noted 
that  plates  parallel  to  the  most  perfect  pinacoidal  cleavage  yield  no  in- 
terference figure  (bastite  gives  an  acute  bisectrix,  diallage  an  axis  emerg- 
ing to  one  side  of  the  centre  of  the  field  of  view),  and  that  the  inter- 
ference colors  are  considerably  lower  than  in  most  of  the  monoclinic 
pyroxenes.  Cross-sections  giving  rectangular  prismatic  cleavage  show 
in  the  case  of  orthorhombic  pyroxenes  the  point  of  emergence  of  a 
bisectrix,  in  that  of  monoclinic  pyroxenes  an  axis.  Sections  in  the 
macrodiagonal  zone  exhibit  an  axial  bar  in  convergent  light,  which  is 
parallel  to  the  good  pinacoidal  cleavage  in  the  orthorhombic  pyroxenes 
iind  perpendicular  to  it  in  diallage.  Isolated  crystals  and  cleavage 
pieces  show  on  all  faces  in  the  cleavage  zone  parallel  extinction  for  the 
orthorhombic  pyroxenes,  but  partly  parallel  and  partly  inclined  extinc- 
tion for  the  monoclinic  pyroxenes.  The  hardness  is  about  5.5  for  en- 
statite  and  bronzite,  and  6  for  hypersthene. 

The  sp.  gr.  increases  with  the  iron  percentage  from  3.1  for  enstatite 
to  3.5  for  hypersthene.  The  orthorhombic  pyroxenes  are  isomorpfhous 
mixtures  of  MgO,  SiOa  and  FeO,  SiO2,  to  which  may  be  added  incon- 
siderable amounts  of  MnO,  SiOa,  CaO,  SiOa,  and  MgO,  A12O3,  SiOa.  The 
mixtures  in  which  the  percentage  of  FeO  does  not  exceed  $%  are  called 
•enstatite  ;  those  with  as  high  as  14$  FeO,  bronzite  ;  those  higher  in  iron, 
hypersthene.  The  limits  are  entirely  arbitrary,  the  optical  character  of 
hypersthene  commences  for  mixtures  with  about  10$  FeO.  The  ortho- 
rhombic  pyroxenes  in  general  are  not  attacked  by  acids,  and  by  hydro- 
fluoric acid  with  difficulty.  With  hydrofluosilicic  acid  they  yield  abun- 
dant rhombohedral  crystals  of  magnesium  fluosilicate  and  iron  fluosili- 
•cate,  which  are  strongly  refracting  and  strongly  doubly  refracting.  The 
mixtures  poor  in  iron  are  very  difficultly  fusible,  hypersthene  less  so. 

The  massive  enstatites  and  ~bronzites,  which  seldom  exhibit  distinct 
crystal  boundaries,  occur  in  the  norites,  gabbros,  granular  peridotites 
and  the  serpentines  derived  therefrom,  and  in  the  olivine  aggregations 
•of  basaltic  rocks.  They  are  also  found  in  the  oli vine-bearing  members 
of  the  Archaean  and  the  resulting  serpentine  rocks.  Occasionally  they 
form  independent  rock  masses  in  the  Archaean,  or  are  associated 
with  magnesium  carbonates  and  chromite  in  peculiar  deposits.  A 
columnar  structure  is  highly  characteristic  of  enstatite  and  bronzite, 
though  not  constant;  it  seems  to  be  due  to  the  growing  together  of 


OETBORHOMBIG  PYROXENES.  (    '205 

V 

innumerable  thin  prisms  to  form  large  crystalloids  (PL  XVII.  Fig^5)y 
These  minute  prisms  are  not  always  completely  in  contact  throughota^' 
their  length,  but  leave  long  cylindrical  hollows  which  are  often  filled 
with  secondary  iron  ores.  In  longitudinal  sections  it  is  not  always 
easy  to  distinguish  the  long  cavities  from  solid  inclusions,  as  they  ap- 
pear dark  on  account  of  the  high  index  of  refraction  of  their  matrix. 
Moreover  the  ferruginous  members  of  the  bronzite  series  carry  the 
same  metallic  to  sub-metallic  scales  and  particles  which  are  so  charac- 
teristic of  massive  hypersthene ;  they  also  have  the  same  arrangement 
as  in  the  last-named  mineral.  Inclusions  of  magnetite,  chromite,  pico- 
tite,  and  other  older  secretions  are  frequent,  but  fluid  inclusions  are 
quite  rare.  A  lamellar  intergrowth  of  massive  enstatite  and  bronzite 
with  monoclinic  pyroxene  is  very  widespread,  and  is  often  first  recog- 
nized in  polarized  light  (PL  XVII.  Fig.  6).  '  The  orthorhombic  and 
monoclinic  lamellae  are  so  placed  with  reference  to  one  another  that  the 
face  ooPoo  (010)  of  the  latter  coincides  with  oopoo  (100)  of  the  former, 
that  is,  the  acute  and  obtuse  prism  angles  have  the  same  position  in 
both  minerals;  ccP&>  (010)  or  one  face  of  ooP  (110)  serves  as  the 
composition  plane.  Since  the  orthodiagonal  zone  of  the  monoclinic 
lamellae  coincides  with  the  brachydiagonal  zone  of  the  orthorhombic 
ones,  the  extinction  in  sections  in  these  zones  is  the  same  in  both  min- 
erals, and  hence  their  intergrowth  is  not  noticeable  in  parallel  polarized 
light.  In  other  sections  in  the  zone  of  the  vertical  axis  the  extinction 
is  parallel  to  the  cleavage  in  the  orthorhombic  lamellae,  and  inclined  to 
it  in  the  monoclinic.  The  intergrowth  sometimes  extends  to  complete 
mutual  penetration,  and  the  lamellae  may  sink  to  immeasurable  thinness. 

Enstatite  and  bronzite  are  found  in  well-developed  crystals  in 
many  porphyritic  rocks,  always  accompanied  by  monoclinic  pyroxene, 
less  frequently  by  hornblende  and  biotite;  they  also  occur  in  the 
trachytes  and  andesites.  The  microstructure  of  these  occurrences 
differs  entirely  from  that  of  the  massive  forms.  The  parallel  compo- 
sition of  small  individuals  is  wanting,  and  with  it  the  columnar 
structure  and  tubular  hollows  parallel  to  the  vertical  axis;  the  lamel- 
lar penetration  with  monoclinic  augite  is  also  wanting,  although  the 
parallel  growth  of  distinct  crystals  of  both  kinds  occurs,  partly  as 
lateral  juxtaposition,  partly  as  the  surrounding  of  orthorhombic  crys- 
tals by  monoclinic.  Microlitic  scales  also  are  absent,  but  glass  inclu- 
sions either  round  or  in  the  form  of  their  host  are  frequent  and 
characteristic. 

The  radially  fibrous  aggregation  of  bronzite  and  enstatite  in  the 
chondri  of  many  meteorites  is  wholly  unique. 


206         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

Enstatite  and  bronzite  are  comparatively  susceptible  to  the  reagents 
occurring  in  nature.  The  massive  occurrences  alter  to  parallel  fibrous 
aggregates,  a  process  which  sets  in  from  the  cross  cracks  and  cleavage ; 
the  alteration  product  being  bastite  and  serpentine,  much  more  rarely 
talc.  The  bastite  alteration  is  not  uncommon  in  the  crystals  of  the 
porphyritic  and  andesitic  rocks  (PI.  XVIII.  Fig.  1) ;  here  the  process 
continues  until  the  silicate  is  broken  up,  and  there  results  pseudo- 
znorphs  of  a  mixture  of  carbonates  with  limonite  and  quartz. 

Hypersthene  forms  poorly  defined  masses  in  the  more  basic  mem- 
bers of  the  granular  eruptive  rocks  (gabbro,  norite),  and  appears  in 
thin  prismatic  crystals  in  the  porphyrites,  trachytes,  andesites,  and 
lavas.  It  is  to  be  remarked  that  it  is  entirely  absent  from  the  normal 
members  of  the  Archaean;  where  it  is  met  with  (in  trap  granulites, 
labradorites,  and  amphibolites)  there  are  generally  grounds  for  con- 
sidering the  rocks  as  regionally  metamorphosed  eruptive  masses.  The 
massive  hypersthene  of  the  gabbros  and  norites  occasionally  possess 
the  vertical  fibration  due  to  the  parallel  growth  of  thin  prisms,  as  well 
as  the  lamellar  intergrowth  and  penetration  with  monoclinic  pyroxene. 
Hypersthene  is  also  intergrown  with  hornblende  lamellae,  so  .that 
the  faces  ooPoo  (100)  of  the  latter  mineral  coincide  with  the  faces 
ooPoo  (010)  of  the  former.  Massive  hypersthene  encloses,  besides 
the  older  minerals  associated  with  it  (magnetite,  apatite,  zircon,  olivine, 
biotite),  tabular  microlitic  interpositions,  which  are  so  frequent  as  to 
be  almost  constant.  They  lie  in  three  different  directions,  with  their 
tabular  faces  parallel  to  the  principal  cleavage  face,  ooPao  (010),  of 
the  hypersthene.  The  plates  are  approximately  rhombic,  almost 
rectangular  or  irregular,  seldom  perfectly  straight-edged,  grading 
into  short  prisms,  and  appear  as  very  thin,  opaque  strips  or  points 
in  all  sections  which  are  not  parallel  to  the  plane  of  the  most  perfect 
cleavage.  Their  color  according  to  their  thickness  is  dark  brown 
to  opaque,  reddish  brown,  light  brown,  yellowish,  grayish  white  to 
almost  colorless.  The  thicker  opaque  plates  have  a  metallic  habit, 
and  even  the  transparent  ones  have  a  submetallic  habit  in  reflected 
light.  These  plates  and  prisms,  to  which  hypersthene  as  well  as 
bronzite  and  diallage  owe  the  metallic  sheen  of  their  principal  cleavage 
face,  lie  with  their  longer  axis  usually  at  right  angles  to  the  vertical 
axis,  less  frequently  parallel  to  it  or  inclined  at  about  30°  (PI.  VII. 
Fig.  5).  In  reflected  light  these  thin  plates  exhibit  the  most  brilliant 
Newton  colors.  Their  exact  nature  is  not  known  :  they  are  consid- 
ered primary  interpositions  by  some,  and  referred  to  titanic  iron  and 
brookite ;  by  others  they  are  thought  to  be  secondary  infiltration  prod- 


ORTHORHOMBIC  PYROXENES.  207 

nets.  It  seems  probable  that  they  are  not  always  of  the  same  nature, 
being  undoubtedly  primary  in  some  instances  and  secondary  in  others. 
Thus  Trippke*  and  Kosmann  f  considered  them  secondary  infiltration 
products,  and  found  them  isotropic  in  diallage ;  they  considered  them 
opal,  and  referred  their  shape  to  that  of  their  host.  Judd  J  has  re- 
cently made  a  special  study  of  these  and  similar  inclusions  in  the 
pyroxenes  and  feldspars  of  the  peridotites  of  Scotland,  and  has  arrived 
at  the  conclusion  that  in  these  occurrences  the  orderly  arranged  in- 
clusions are  not  definite  chemical  compounds,  but  are  mixtures  of 
various  oxides  in  a  more  or  less  hydrated  condition,  such  as  hyalite, 
opal,  gothite,  and  limonite.  These  have  been  deposited  in  negative 
crystal  cavities,  which  they  may  fill  completely  or  only  partially  :  in  the 
first  case  their  boundaries  correspond  to  the  crystal  form  of  the  en- 
closing mineral ;  in  the  second  they  are  irregular.  The  cavities  have 
been  formed  along  certain  definite  planes  within  the  original  crystal, 
which  correspond  to  planes  of  least  resistance  to  chemical  action 
called  solution  planes  ;  and  the  solvent  has  acted  under  the  influence 
of  great  pressure,  and  therefore  he  concludes  that  this  process  of 
charging  a  mineral  with  definitely  oriented  inclusions,  which  he  has 
termed  schillerization,  is  a  secondary  process,  which  only  takes  place 
at  considerable  depths  beneath  the  surface  of  the  earth. 

On  the  other  hand,  G.  H.  Williams  §  has  called  attention  to  the 
fact  that  similar  inclusions  exist  in  certain  feldspars,  hypers thenes, 
etc.,  under  conditions  which  clearly  indicate  that  in  these  particular 
instances  they  are  primary  bodies,  which  were  formed  contempora- 
neously with  their  hosts,  and  cannot  be  considered  as  the  results  of 
subsequent  alteration,  and  that  they  may  be  distinguished  from  those 
of  secondary  origin  in  many  cases,  but  not  in  all. 

The  hypersthene  prisms  of  the  porphyritic  rocks  have  not  the 
niicrostructure  of  the  massive  occurrences,  but  possess  throughout  the 
relations  of  the  geologically  equivalent  bronzite  crystals:  however, 

*  P.  TRIPPKE,  Ueber  den  Enstatit  aus  den  Olivinknollen  des  Gr5ditzberges. 
N.  J.  B.  1878.  673-681. 

t  B.  KOSMANN,  Ueber  das  Schillern  und  den  Dichroismus  des  Hypersthens. 
N.  J.  B.  1869.  532. 

t  J.  W.  JUDD,  On  the  Tertiary  and  older  Peridotites  of  Scotland.  Quart. 
Journ.  Geol.  Soc.  Aug.  1885. 

-  On   the   Relations  between   the    Solution-planes   of    Crystals  and  those   of 
Secondary  TwiDning,  etc.    Mm.  Mag.  Vol.  VII.  pp.  81-92.     1886. 

§  G.  H.  WILLIAMS,  Peridotites  of  the  "  Cortland  Series"  on  the  Hudson  River, 
near  Peekskill,  N.  Y.  Am.  Journ.  Sci.  Vol.  XXXI.  Jan.  1886. 

-  The  Norites  of  the  "  Cortland  Series,"  etc.    Am.  Journ.  Sci.  Vol.  XXXIII. 
Feb.  1887  and  March,  1887. 


208         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

they  sometimes  enclose  fine  needles  with  a  metallic  habit,  which  are  not 
found  in  the  bronzites.  Bronzite  and  hypersthene  are  widely  dissemi- 
nated in  the  trachytic  and  andesitic  eruptive  rocks.*  Hypersthene  is 
rather  rare  in  the  Archaean  ;  when  it  occurs  here  it  is  in  more  or  less 
well-defined  crystals. 

Hypersthene  withstands  decomposition  much  better  than  bronzite 
and  enstatite  do.  It  very  seldom  alters  to  bastite.  Its  alteration  to 
limonite  is  not  uncommon  in  the  very  ferruginous  occurrences  in  an- 
desites.  The  hypersthene  of  the  gabbro  rocks  very  often  alters  into 
amphibole  (actinolite  and  ordinary  hornblende),  f 

Appendix. — With  the  alteration  of  enstatite  and  bronzite  to  bastite, 
which  may  commence  with  the  taking  on  of  water,  there  appear  very 
rapid  changes  in  the  ellipsoid  of  elasticity  of  these  minerals,  and  those 
axes  of  elasticity  coincident  with  the  horizontal  axes  change  places  with 
one  another.  Hence  the  optical  scheme  becomes  c  =  c,  a  =  b,  T)  =  a; 
the  axial  plane  now  stands  perpendicular  to  the  principal  cleavage  paral- 
lel to  the  brachypinacoid :  it  lies  in  the  macropinacoid,  and  the  macro- 
diagonal  is  the  negative  bisectrix.  The  axial  angle  is  generally  quite 
large,  but  varies  considerably  in  cleavage  plates  taken  from  the  same 
material.  Diaclasite  represents  such  a  stage  in  the  alteration  of  ensta- 
tite and  bronzite  to  bastite.  Cleavage  plates  of  this  mineral  assume  a 

*  J.  NIEDZWIEDZKI,  Andesit  von  St.  Egidi  in  Steiermark.     T.  M.  M.  1872.  253. 

J.  PETERSEN,  Mikroskop.  u.  chem.  Untersuchungen  am  Enstatitporphyrit  aus 
den  Cheviot  Hills.  Kiel.  1884. 

J.  J.  H.  TEALL,  On  the  Cheviot  Andesites  and  Porphyrites.  Geol.  Mag.  (2) 
X.  1883.  No.  225.  226'.  228. 

P.  TELLER  and  C.  VON  JOHN,  Geologisch  petrographische  Beitra'ge  zur  Kennt- 
niss  der  dioritischen  Gesteine  von  Klausen  in  Stid-Tyrol.  Jahrb.  d.  k.  k.  geologi- 
schen  Reichsanstalt.  1882.  XXXII.  589-684. 

WHITMAN  CROSS,  On  hypersthene  andesite.  Amer.  Journ.  1883.  XXV.  No.  146. 
139-144. 

ARNOLD  HAGUE  and  J.  P.  IDDINGS,  Notes  on  the  volcanic  rocks  of  the  Great 
Basin.  Amer.  Journ.  1884.  XXVII.  No.  162,  and  Notes  on  the  volcanoes  of  Northern 
California,  Oregon,  and  Washington  Territory,  ibidem.  1883.  XXVI.  September. 
222-235. 

H.  H.  REUSCH,  Vulkanische  Asche  von  den  letzten  Ausbrilchen  in  der  Sunda- 
strasse.  N.  J.  B.  1884.  I.  45. 

In  H.  ABICH,  Geologische  Forschungen  in  den  Kaukasuslandern.  II.  Wien. 
1882.  329-364. 

f  F.  BECKE,  In  olivine  gabbro  of  Laugenlois,  Lower  Austria.  T.  M.  P.  M.  IV. 
1882.  355. 

G.  H.  WILLIAMS,  Preliminary  notice  of  the  Gabbros  and  associated  Hornblende 
rocks  in  the  vicinity  of  Baltimore.  Johns  Hopkins  University  circulars.  1884. 
No.  30;  also  Bull.  28,  U.  S.  Geol.  Survey.  1886.  42. 

ARNOLD  HAGUE  and  J.  P.  IDDINGS,  Notes  on  the  volcanic  rocks  of  the  Great 
Basin.  Am.  Journ.  Sci.  Vol.  XXVII.  June.  1884.  459. 


BASTITE.  209 

metallic  lustre  with  a  brass-yellow  color;  the  hardness  and  specific 
gravity  decrease  with  the  alteration.  While  diaclasite  may  be  easily 
distinguished  from  enstatite  and  bronzite  by  the  position  of  the  optic 
axes,  it  must  be  distinguished  from  bastite  by  its  specific  gravity, 
which  for  bastite  is  2.74,  for  diaclasite  is  over  2.8. 

Bastite. 

Literature. 

R.  VON  DRASCHE,  Ueber  Serpentine  und  serpentinahnliche  Gesteine.     T.  M.  M. 

1871.  I.  10-12. 

E.  REUSCH,  Ueber  das  Schillern  gewisser  Krystalle.  Pogg.  Ann.  1863.  CXX.  115. 
G.  TSCHEKMAK,  Ueber  die  mikroskopische  Unterscheidung  der  Mineralien  aus  der 

Augit-,  Amphibol-  und   Biotitgruppe.      S.  W.  A.  1869.  LIX.   1.  Abthl.  1-12. 
—Ueber  Pyroxen  und  Amphibol.     T.  M.  M.  1871.  I.  20, 
C.  E.  WETSS,   Beobachtungen  und  Untersuchungen  liber  den  Schillerspath  von 

Todtmoos.    Pogg.  Ann.  1863.  CXIX.  459. 

Bastite  or  "  schiller-spar "  is  always  a  pseudomorph  after  an  ortho- 
rhombic  pyroxene  poor  in  iron.  Hence  it  has  no  crystal  form  of  its 
own,  but  always  appears  in  that  of  enstatite  and  bronzite  ;  conse- 
quently it  is  found  massive  in  lamellar  crystalloids  with  distinct  verti- 
cal fibration  in  the  granular  massive  rocks  and  their  derivatives,  and 
in  prismatic  crystals  in  the  porphyritic  or  andesitic  rocks  which  in  a 
fresh  condition  bear  bronzite  and  enstatite.  The  prisms  also  exhibit  a 
very  pronounced  vertical  fibration,  the  fibres  extending  from  one  trans- 
verse crack  to  another,  and  not  being  continuous  throughout  the  entire 
length  of  the  prism  (PL  XYIII.  Fig.  1).  In  the  alteration  of 
massive  bronzite  to  bastite  the  original  microstructure  is  almost  com- 
pletely retained,  but  the  glass  inclusions  in  the  porphyritic  crystals 
are  generally  destroyed  in  the  process  of  alteration.  Along  the  trans- 
verse cracks,  that  is,  between  the  different  systems  of  bastite  fibres, 
there  are  often  deposited  iron  ores  (magnetite,  limonite),  which  may  be 
derived  from  the  iron  contained  in  the  primary  mineral ;  in.  other 
cases  these  cracks  are  filled  with  confusedly  fibrous  serpentine. 

In  bastite  the  cleavage  parallel  to  the  brachypinacoid  is  much  more 
perfect  than  in  the  original  mineral,  but  the  cleavage  parallel  to  oo^P 
(110)  is  less  distinct.  The  lustre  on  cleavage  faces  of  the  massive 
varieties  is  metallic,  but  more  silky  for  the  well-crystallized  varieties. 
The  hardness  is  less  than  for  the  original  mineral,  3.5-4. 

Bastite  becomes  transparent  with  a  light-yellowish  or  light-greenish 
color,  and,  optically,  behaves  most  uniformly.  The  extinction  in 
longitudinal  sections  lies  parallel  and  at  right  angles  to  the  axis  of  the 
fibres.  If  the  fibres  do  not  lie  exactly  parallel,  the  optical  behavior, 


210 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


c=c 


Fig.  70 


naturally,  cannot  be  uniform.  The  axial  plane  lies  in  the  macropina- 
coid,  and  b  is  the  negative  bisectrix.  The  size  of  the  axial  angle  varies 
within  wide  limits  from  20°  to  nearly  90°;  hence  the  cleavage  plates 
must  be  placed  in  oil  to  observe  the  axial  figure.  Dispersion  p  >  v. 
The  mean  index  of  refraction  is  lower  than  for  bronzite,  about  1.5-1.6. 

The  double  refraction  is  weak.  The  pleo- 
chroism  is  weak  ;  rays  vibrating  parallel  to 
the  fibres  are  the  most  strongly  absorbed, 
but  it  requires  rather  thick  sections  to  make 
out  the  difference.  Its  behavior  in  con- 
vergent light  is  the  surest  means  of  distin- 
guishing it  from  the  original  mineral ;  for 
its  diagnosis  with  reference  to  diaclasite, 
see  tinder  the  latter  mineral.  The  optical 
scheme  of  bastite  is  given  in  Fig.  TO. 
Sp.  gr.  2.6-2.8,  considerably  lower  than  that  of  enstatite,  bronzite, 
and  diaclasite.  The  chemical  composition  of  pure  bastite  appears  to 
be  the  same  as  that  of  serpentine,  H2O,  3MgO,  2SiO2  -|-  aq.,  with  a 
variable  replacement  of  MgO  by  FeO  ;  small  quantities  of  chromium 
are  referred  to  inclusions  of  chromite  or  picotite.  Upon  being  heated 
to  redness,  bastite  becomes  cloudy  and  dirty  grayish  black  to  brownish. 
It  gelatinizes  with  difficulty  with  hydrochloric  acid,  easily  with  sul- 
phuric acid,  especially  at  a  high  temperature.  Bastite  has  no  inde- 
pendent geological  position  ;  it  is  always  secondary,  replacing  enstatite 
and  bronzite.  It  is  not  definitely  known  whether  other  pyroxenes,  as 
diallage,  can  be  altered  to  bastite,  but  it  seems  quite  probable. 

The  Group  of  Orthorhoinbic  Amphiboles. 
Literature. 

H.  SJOGREN,  Forekomsten  af  Gedrit  sasom  vasendtlig  bestandsdel  i  nagra  norska 
och  finska  bergarter.  Kongl.  Vetensk.  Akad.  Forhandl.  Stockholm.  1882. 
No.  10.  5-11. 

G.  TSCHERMAK,  Ueber  Pyroxen  und  Amphibol.     T.  M.  M.  1871.  I.  37. 

The  orthorhombic  amphiboles  which  occur  as  rock  constituents  are 
antJiophyllite  and  gedrite.  Both  appear  in  prismatic  and  lamellar 
aggregations,  which  never  exhibit  terminal  crystallographic  boundaries, 
but  frequently  those  in  the  prism  zone,  ooPc»  (100)  and  o>P  (110). 
The  prism  angle  lies  between  124°  and  125°.  Hence  the  sections 
are  irregular :  those  parallel  to  the  prism  axis  are  lath-shaped  to  broad 
and  tabular ;  those  at  right  angles  to  it  are  often  six-sided,  or  acutely 


ORTH011HOMB1C  AMPHIBOLES. 


211 


rhombic  when  ooPoo  (100)  is  wanting.  The  composition  of  the  larger 
masses  of  separate,  thin,  prismatic  individuals  produces  a  fine  vertical 
striation  "quite  analogous  to  that  of  brorizite  and  enstatite.  In  conse- 
quence of  the  separate  individuals  not  growing  together  exactly 
parallel,  there  arises  a  more  or  less  divergent  arrangement  which  in 
some  instances  becomes  almost  radial. 

The  cleavage  is  very  perfect  parallel  to  ccPao  (100),  perfect  parallel 
to  OD P  (110),  and  scattered  but  sharp  cracks  indicate  an  imperfect 
cleavage  parallel  to  ooPoo  (010).  Moreover,  there  is  a  transverse 
parting  approximately  parallel  to  oP  (001),  which  is  never  perfectly 
plain.  In  sections  in  the  prism  zone  all  the  cleavage  cracks  are 
parallel ;  at  right  angles  to  this  they  form  acute  rhombs  of  55°  to  56°, 
with  diagonal  cross  cracks.  The  orthorhombic  amphiboles,  ac- 
cording to  their  percentage  of  iron,  become  transparent  and  almost 
colorless  to  yellowish  and  reddish  brown  and  yellowish  green. 
The  mean  index  of  refraction  has  been  determined  by  Des  Cloiz- 
eaux,  /?p  =  1.636.  The  double  refraction  is  very  strong ;  hence 
the  interference  colors  are  high  in  sections  which  are-not  perpendicular 
to  an  optic  axis :  even  in  quite  thin  sections  the  colors  are  red,  blue, 
and  green,  which  distinguish  the  mineral  very  well  from  orthorhom- 
bic pyroxenes  in  longitudinal  sections.  The  extinction  in  sections  in 
the  three  principal  zones  naturally  lies  parallel  and  perpendicular  or 
diagonal  to  the  cleavage  cracks.  The  plane  of  the  optic  axes  lies  in 
the  brachypiriacoid,  and  cleavage  plates  parallel  to  ooPob  (100)  yield 
an  axial  figure  in  oil,  with  the  bisectrix  emerging  normally.  The  opti- 
cal scheme  is  #  =  a,  b  =  b,  c  =  C.  In  some  occurrences  (anthophyllite)  c 
is  the  acute  bisectrix,  and  the  character  of  the  double  refraction  is 
positive,  and  2JTa  —  81°-82° ;  in  other  occur-  ^ 

rences,  and  especially,  as  it  appears,  for  the  alu- 
minous varieties  (gedrite),  #  is  the  acute  bisec- 
trix ;  the  character  is  negative,  and  2Ha  varies 
from  47°-82°,  even  in  cleavage  pieces  from  the 
same  occurrence.  The  dispersion  is  independent 
of  the  character  of  the  double  refraction,  p  <  v 
about  6,  p  >  v  about  d ;  but  it  is  sometimes 
quite  weak,  and  difficult  to  determine.  The 
pleochroism  is  dependent  on  the  depth  of  the 
color ;  the  rays  vibrating  parallel  to  the  axis  of 
the  prism  are  light  yellowish  brown  or  greenish 
to  colorless  ;  those  at  right  angles  to  it  are  clove- 
brown.  Absorption  c  <  a  =  b.  The  optical  scheme  is  given  in  Fig.  71. 


^<PA  c 

-'i  \, 

\ 

\ 

j 

\ 

1 

!«=a 

i 

i 

.  —  -  —  J 

E^ig;.  TO. 

212         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

H.=  5.5.  Sp.  gr.  =  3.15-3.24.  The  chemical  composition  of  an- 
thophyllite is  (Mg,  Fe)O,  SiO2 ;  gedrite  also  contains  an  aluminous 
molecular  compound,  MgO,  A12O3,  SiO2,  in  variable  amount.  The  per- 
centage of  A12O3  rises  to  13.5$.  Neither  mineral  is  noticeably 
attacked  by  acids.  Cleavage  fragments  are  very  difficultly  fusible. 

Anthophyllite  and  gedrite  possess  no  constant  microstructure.  They 
are  apt  to  enclose  hematite  plates,  magnetite  grains,  spinel  octahedrons 
(picotite),  and  biotite  plates.  The  latter  lie  with  their  tabular  faces  in 
the  cleavage  faces  of  v>P  (110),  rarely  in  those  parallel  to  &P<x>  (100). 
The  same  microlitic  forms  which  were  described  for  hypersthene  are 
sometimes  found  as  interpositions  in  these  minerals  also.  A  regular 
lamellar  intergrowth  with  monoclinic  amphibole  (actinolite)  is  not 
uncommon ;  both  kinds  of  amphibole  then  have  the  axes  c  and  &  in 
common  :  hence  this  intergrowth  is  only  noticeable  in  parallel  polar- 
ized light  on  sections  which  do  not  lie  in  the  zone  oP:  oo^Pw,  and 
in  which  the  lamellae  of  monoclinic  amphibole  extinguish  obliquely 
to  the  cleavage  or  to  its  diagonal.  On  cleavage  plates  parallel  to 
ooPoo  (100)  the  monoclinic  lamellae  are  recognized  by  the  oblique 
emergence  of  an  axis,  while  in  the  orthorhombic  lamellae  a  bisectrix 
emerges  perpendicularly. 

Anthophyllite  and  gedrite  are  Archaean  minerals  belonging  especially 
to  the  hornblende  gneisses  and  hornblende  schists,  in  which  they  are 
sometimes  disseminated  as  an  essential  constituent,  sometimes  are 
grouped  together  in  radiaAaggregations.  Anthophyllite  is  often  quite 
abundant  in  olivine  serjjntines,  generally  accompanied  by  bastite. 

Nothing  is  known'  concerning  the  processes  of  decomposition  of 
anthophyllite. 

Olivine. 

Literature. 
E.  COHEN,  Ueber  Laven  von  Hawaii  und  einigen  andern  Inseln  des  grossen  Oceans 

nebst  einigen  Bemerkungen  tlber  glasige  Gesteine  im  allgemeinen.      N.  J.  B. 

1880.  II.  23-62. 
R.  HAGGE,  Mikroskopische  Untersuchungen  uber  Gabbro  und  verwandte  Gesteine. 

Kiel..  1871. 

E.  KALKOWSKY,  Ueber  Olivinzwillinge  in  Gesteinen.     Z.  X.  1885.  X.  17-24. 

F.  KBEUTZ,  Ueber  Vesuvlaven  von  1881  und  1883.     T.  M.  P.  M.  1885.  VI.  142-148. 
H.  ROSENBUSCH,  Petrographische  Studien  an  Gesteinen  des  Kaiserstuhls.      N.  J.  B. 

1872.  59  sqq. 

G.  TSCHEKMAK,  Beobachtungen  uber  die  Verbreitung  des  Olivins  in  den  Felsarten. 

S.  W.  A.  1867.  LVI.  Juli. 
F.  ZIRKEL,  Untersuchungen  uber  die  mikroskopische  Zusammensetzung  und  Struktur 

der  Basaltgesteine.    Bonn.  1870.  55-67. 
—  Geologische  Skizzen  von  der  Westkuste  Schottlands.     Z.  D.  G.  G.  1871.  XXIII. 

59-95. 


OLIVINE. 


213 


Olivine  forms  either  well-developed  crystals  and  incipient  forms  of 
growth  ;  or  irregularly  defined  rounded  or  angular  grains ;  or,  finally, 
granular  aggregates.  The  crystals  have  the  habit  of  Figs.  72  and  73, 


-.  7,3 


with  the  faces  a—  ooP5o  (100),  I  =  ooP<x>  (010),  c  =  oP  (001),  m  = 
ooP(110),  s  =  ooP2  (120),  d  =  P&  (101),  A  =  Poo  (Oil),  It  =  2Poo 
(021),  0=P(111).  The  faces  0P(001)  are  usually  very  small,  often 
entirely  wanting ;  then  the  sections  in  the  vertical  zone  are  six-sided, 
otherwise  they  are  octagonal.  Individuals  of  microscopic  dimensions 
sometimes  appear  monosymmetric  on  account  of  a  kind  of  hemimorph- 
ism  (PL  XVIII.  Fig.  2).  The  incipient  forms  of  growth  are  extremely 
manifold ;  besides  simple  forked  forms  (PL  III.  Fig.  1),  there  are  deli- 
cate forms  of  bisymmetric  or  hemimorphic  monosymmetric  character, 
some  of  which  are  represented  in  Fig.  74.  Twins  occur,  the  twinning 


ing.  74: 

plane  being  Poo  (Oil).  It  is  often  repeated,  the  individuals  penetrating 
one  another,  and  the  boundaries  of  the  twins  being  difficultly  distin- 
guishable in  ordinary  light  (PL  XVIII.  Fig.  3).  The  twins  are  only 
determined  with  certainty,  optically,  in  sections  parallel  to  the  macro- 
pinacoid  when  the  vertical  axis  of  the  twinned  individuals  and  their 
extinctions  make  an  angle  of  68°  48'  with  one  another. 


214  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

The  outline  of  the  crystal  sections  often  exhibits  a  decided  rounding- 
with  variously  shaped  loops,  the  result  of  corrosion  by  the  magma  out 
of  which  they  crystallized.  The  deformations  extend  to  the  complete 
obliteration  of  the  original  crystal  form ;  thus  arise  the  isolated  olivine 
grains.  In  many  rocks  (Iherzolites,  olivinite,  and  olivine  schists)  in 
which  olivine  forms  granular  aggregations  it  appears  never  to  have 
reached  the  development  of  a  crystal  form.  The  grains  are  not 
deformed  crystals,  but  those  whose  development  has  been  hindered. 
In  meteorites  olivine  exhibits  the  chondritic  form. 

The  cleavage  of  olivine  parallel  to  oo^Poo  (010)  is  shown  in  thin 
sections  by  more  or  less  distinct  parallel  cracks,  which  are  seldom 
abundant,  and  often  wedge  out  in  the  crystal.  Still  less  frequent  and 
irregular  are  the  cracks  corresponding  to  the  cleavage  parallel  to  ooPoo 
(100)  ;  the  cleavage  parallel  to  oP  (001)  is  at  least  indicated  in  thin  sec- 
tion. It  often  appears  as  though  the  ferruginous  varieties  (hyalosiderite 
and  fayalite)  possessed  more  perfect  cleavage  and  less  of  the  corrosive 
deformation  of  the  crystal  outline.  Besides  the  cleavages,  there  is 
always  an  irregular  fracturing  of  the  crystal,  which  appears  to  increase 
with  the  alteration  into  serpentine. 

Olivine  is  transparent  and  nearly  colorless  to  greenish  white,  with  a 
high  iron  percentage;  and  under  certain  conditions  it  is  red  to  reddish 
brown.  The  index  of  refraction  is  high ;  hence  the  relief  is  consider- 
able and  the  surface  decidedly  rough  (PI.  XVIII.  Fig.  4).  The  double 
refraction  is  very  strong,  and  the  interference  colors  even  in  quite 
thin  sections  are  of  the  2d  and  3d  order.  Des  Cloizeaux  determined 
orna  =  1.661,  /3na  =  1.678,  yna  =  1.697;  Michel-Levy  found  on  artificial 
fayalite  y  —  a  =  0.043.  The  plane  of  the  optic  axes  lies  in  oP  (001)  and 
a  is  the  positive  bisectrix ;  hence  a  =  l,  b  =  ft,  c  =  b  (Fig.  72).  The 
axial  angle  is  large,  2  Vna  =  87°  46';  the  dispersion  is  weak,  p<v  about  a. 
In  consequence  of  the  large  axial  angle  unsymmetrical  sections  may 
show  an  extinction  which  is  quite  oblique  to  the  edges  and  cleavages, 
while  sections  from  the  principal  zones  extinguish  parallel  and  perpen- 
dicular to  the  axes  of  these  zones  and  to  the  cleavage.  The  optical 
behavior  may  occasionally  lead  to  confusion  with  colorless  pyroxenes 
of  the  monoclinic  system.  It  is  to  be  remarked,  however,  that  the 
latter  possess  two  equivalent  cleavages,  while  those  of  olivine  are 
unequal ;  further,  that  in  the  three  principal  zones,  when  the  cleavages 
intersect,  the  extinction  for  olivine  lies  parallel  and  perpendicular  to 
them,  while  for  pyroxene  it  is  diagonal.  In  general  there  is  no  appre- 
ciable pleochroism  ;  but  in  the  red  olivines  rays  vibrating  parallel  to  & 
are  less  absorbed  than  those  parallel  to  a  and  5. 


OLIVINE. 


215 


H.  =  6.5-7.  Sp.  gr.= 3.3-3.45.  Chemical  composition  (Mg,  Fe)O, 
SiO3.  The  more  ferruginous  varieties  are  called  hyalosiderite,  those 
with  predominating  iron  percentage  are  called  f ayalite.  Olivines  which 
are  not  too  poor  in  iron  become  permanently  red  upon  being  heated  to 
redness,  and  then  exhibit  more  or  less  distinct  pleochroism.  In  thin  sec- 
tion olivine  is  attacked  but  slowly  by  cold  hydrochloric  acid,  but  more 
rapidly  when  heated,  with  the  separation  of  gelatinous  silica ;  ferrugin- 
ous varieties  gelatinize  more  easily  than  those  poor  in  iron,  and  sulphu- 
ric acid  acts  more  energetically  than  hydrochloric.  This  gelatinization 
may  often  be  used  to  distinguish  olivine  from  other  minerals  of  similar 
habit,  by  coloring  the  surface  coating  as  already  described.  The 
reaction  for  Mg  and  Fe  is  obtained  with  hydrofluosilicic  acid,  when 
the  surface  of  the  mineral  becomes  covered  with  etched  figures  which 
exhibit  acutely  rhombic  outlines. 

Three  kinds  of  olivine  may  be  distinguished  :  olivine  of  the  granu- 
lar eruptive  rocks,  olivine  of  the  porphyritic  eruptive  rocks,  and  olivine 
of  the  crystalline  schists. 

The  olivine  of  the  granular  eruptive  rocks,  as  it  occurs  in  olivine 
diabases,  olivine  gabbros,  olivine  norites  and  peridoites,  and  as  it  is 
found  in  the  older  segregations  of  the  volcanic  rocks,  exhibits  no 
perfectly  regular  crystallographic  boundary.  It  is  always  evident  that 
it  is  older  than  the  other  silicates  accompanying  it,  and  from  the  nature 
of  the  decomposition  processes  it  is  clear  that  very  ferruginous  varieties 
do  not  occur.  These  olivines  enclose  crystals  of  magnetite,  ilmenite, 
apatite  and  chromite ;  in  many  gabbro  and  norite  occurrences  they  are 
crowded  with  needles  and  tabular  microscopic  interpositions,  which  are 
arranged  parallel  to  the  three  principal  sections  and  possess  a  metallic 
habit :  they  appear  to  be  a  titaniferous  iron  compound.  Fluid  inclu- 
sions are  not  infrequent. 

The  olivine  of  the  porphyritic  eruptive  rocks  (melaphyres,  basalts, 
basanites,  nepheline  and  leucite  basalts,  limburgites)  belong  to  the 
secretions  of  the  first  period  of  consolidation;  their  crystallization 
immediately  followed  that  of  the  apatites  and  iron  ores,  and  preceded 
that  of  the  mica  and  bisilicates.  Hence  it  appears  in  crystal  forms, 
which,  however,  are  very  often  disturbed  by  subsequent  corrosion. 
Pockets  and  inclusions  of  the  ground  mass  are  very  characteristic  of  this 
variety  of  olivine  (PL  XYIII.  Fig.  5),  as  are  also  glass  inclusions  of 
manifold  shapes,  irregular  inclusions  of  fluids,  among  which  is  liquid 
carbon  dioxide,  and  interpositions  of  the  older  minerals  associated  with 
it,  especially  magnetite,  ilmenite,  chromite  and  picotite.  Ferruginous 
varieties  are  very  frequent  in  these  rocks.  Here  also  belong  the 


216  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

skeleton  crystals  and  incipient  forms  of  growth  which  are  found  in 
glassy  rocks,  which  only  reached  incomplete  crystalline  development ; 
they  often  enclose  remarkably,  large  portions  of  the  rock  glass  arranged 
symmetrically  (PL  XVIII.  Fig.  2).  Indications  of  a  second  generation 
of  olivine  crystals  in  the  porphyritic  eruptive  rocks  are  rarely  found. 
Olivine  appears  as  an  accessory  constituent  even  in  rocks  of  the 
trachytic  and  andesitic  series  quite  rich  in  alkalies  and  poor  in  bivalent 
bases,  and  often  stands  peculiarly  correlated  to  the  orthorhombic 
pyroxenes. 

In  the  Archsean  rocks  olivine  is  sometimes  an  accessory  constituent, 
sometimes  an  essential  constituent  in  a  series  of  rocks  which  occurs  in 
the  form  of  an  inclusion,  and  consists  at  one  end  of  dolomitic  limestone 
and  dolomite,  while  at  the  other  it  is  wholly  made  up  of  silicate  rocks 
rich  in  magnesia  (olivine  rocks),  in  which  no  magnesia  or  iron  carbon- 
ates are  found,  or  in  which  there  are  only  traces  of  them.  In  this 
series  belong  many  amphibolites,  pyroxenites,  and  eklogites.  The 
olivine  of  these  rocks  has  the  same  habit  as  that  of  the  granular 
eruptive  rocks,  but  the  ore-like  interpositions  are  generally  wanting, 
and  there  occur  only  occasional  inclusions  of  fluids  and  spinels  (chromite, 
picotite,  and  pleonaste). 

Few  minerals  exhibit  such  a  variety  of  alterations  as  olivine. 
Three  different  processes  may  be  distinguished.  The  proper  weather- 
ing, brought  about  by  the  universal  reagents  existing  in  the  surface 
of  the  earth's  crust,  water,  oxygen,  and  carbonic  acid,  leads  to  the  for- 
mation of  carbonates,  silica,  and  limonite  when  the  olivines  are  not  too 
ferruginous ;  mixed  with  these  is  generally  a  variable  amount  of 
serpentine,  so  that  serpentinization  may  possibly  be  the  first  stage  in 
this  process  of  alteration.  The  fact  that  calcite  is  always  present 
among  the  carbonates,  even  to  the  exclusion  of  others,  makes  it  appear 
as  though  this  alteration  was  accompanied  by  a  process  of  impregnation. 
This  process  of  alteration  is  comparatively  rare.  The  most  common 
one  is  the  alteration  of  olivine  to  serpentine :  this  always  starts  from 
the  surface  and  from  cracks  and  leads  to  a  fibration,  at  the  same  time 
with  the  separation  of  the  iron  in  the  form  of  ferric  oxide,  hydrous 
oxide,  and  magnetite.  The  greenish  to  yellowish  green  fibres  stand 
perpendicular  to  the  crystal  boundaries  and  the  cracks.  This  produces  a 
net-like  appearance,  the  strings  of  serpentine  forming  the  web  of  the  net, 
the  meshes  consisting  of  olivine  as  yet  unaltered  (PI.  XYIII.  Fig.  6). 
As  the  process  advances,  new  cracks  form  with  the  increase  in  volume 
xaccompanying  the  serpentinization,  resulting  finally  in  the  complete 
alteration  of  the  olivine.  Although  the  serpentinization  of  olivine  in 


CORDIERITE.  217 

many  cases  may  be  a  simple  act  of  weathering,  yet  in  others  it  is 
probably  due  to  the  action  of  warm  waters.  In  very  ferruginous 
olivines  (hyalosiderites  and  fayalites)  the  alteration,  which  also  com- 
mences from  the  surface  and  from  cracks,  leads  to  ferric  oxide,  which 
passes  secondarily  into  hydrous  oxide  of  iron  (PI.  XIX.  Fig.  1).  At 
first  this  process  often  imparts  to  olivine  a  pleochroisrn  which  did  not 
previously  exist. 

The  third  process  is  the  alteration  of  olivine  to  amphibole ;  it  is 
only  known  to  take  place  in  the  Archaean  rocks,  where  it  occurs  both 
in  the  schistose  and  in  the  eruptive  rocks.  It  can  generally  be  shown 
that  this  alteration  takes  place  through  the  mutual  influence  of  the 
olivine  and  the  adjacent  rock  constituents.  The  new  formation  is  first 
confined  to  the  periphery  of  the  olivine,  and  advances  from  here  in- 
ward. The  needles  of  the  amphibole  minerals,  which  may  be  partly 
referred  to  tremolite,  partly  to  actinolite,  and  partly  to  anthophyllite, 
stand  at  right  angles  to  the  boundary  of  the  olivine,  and  usually  group 
themselves  in  several  zones,  differing  in  color.  To  the  same  group  of 
phenomena  belong  the  alteration  of  olivine  to  a  felt  of  amphibole 
needles,  with  a  slight  admixture  of  serpentine  or  chlorite  and  mag- 
netite; which  has  been  called  pilite  by  Becke  (PL  XIX.  Fig.  2). 
These  alterations,  together  with  the  quite  frequent  formation  of 
amphibole  (actinolite  and  brown  hornblende)  and  of  biotite  from 
serpentinized  olivine  in  the  serpentines  of  the  crystalline  schists,  may 
be  considered  dynamometamorphic. 

Cordierite. 
Literature. 

E.  HUSSAK,  Ueber  den  Cordierit  in  vulkanischen  Auswiirflingen.    S.  W.  A.  1.  Abth. 

1883.  LXXXVII.  April. 
A.  VON  LASAULX,  Ueber  Cordieritzwillinge  in  einem  Auswiirfling  des  Laacher 

Sees.     Z.  X.  1883.  VIII.  76-80. 
J.  SZABO,  Der  Granat  und  Cordierit  in  den  Trachyten  Ungarns.     N.  J.  B.  B.-B.  I. 

1881.  308-320. 
A.  WICHMANN,  Die  Pseudomorphosen  des  Cordierits.     Z.  D.  G.  G.  1874.  XXVI. 

675-701. 

Rock-making  cordierite  sometimes  appears  in  crystals,  often  in 
grains  without  regular  boundaries.  The  crystals  almost  always  exhibit 
the  simple  forms  (Fig.  75)  ooP  (110),  ooP</5  (010),  oP  (001).  The 
prism  angle  is  119°  10'.  PvS  (Oil)  and  \P  (112)  occasionally  occur 
as  narrow  truncations  of  the  combination  edges  of  the  base  with  the 
vertical  faces.  Hence  sections  in  the  prism  zone  are  rectangular, 


218          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

and  at  right  angles  to  it  are  hexagonal  ;  or  the  crystallographic  outlines 
are  wanting.  Twinning  on  the  whole  is  rare;  when  present  it  follows 
the  law  :  the  twinning  plane  is  the  prism  face.  In  cordierite  which 
has  been  exposed  to  volcanic  influences  it  leads  to  penetration  trill- 
ings, in  consequence  of  which  in  a  single  individual 
thin  lamellae  are  often  intercalated  in  place  of  the 
other  individuals.  PL  XIX.  Fig.  3  shows  a  basal 
section  of  a  simple  trilling.  In  the  metamorphic 
c  rocks  there  is  a  development  of  polysynthetic  lamellse 
parallel  to  a  single  prism  face.  Re-entrant  angles  do 
not  occur,  but  with  the  twinning  just  mentioned  there 


cP. 


15  / 


in  >7&  '  ^s  a  Cation  on  cleavage  faces  just  as  on  the  principal 
cleavage  face  of  plagioclase.  The  cleavage  parallel  to 
oo  Po6  (010)  is  very  variable  ;  it  is  occasionally  observed  in  thin  sec- 
tions as  distinct,  parallel  cracks.  But  an  irregular  parting  is  often 
suggested  by  crooked  and  disconnected  cracks,  especially  in  occurrences 
which  are  not  entirely  fresh.  H.  =  7-7.5. 

Cordierite  generally  becomes  transparent  and  colorless,  more  rarely 
yellowish,  blue,  or  violet,  according  to  the  position  of  the  section.  The 
index  of  refraction  and  the  double  refraction  are  weak,  and  strikingly 
similar  to  those  of  quartz,  with  which  mineral  cordierite  may  be  easily 
confounded.  Des  Cloizeaux  determined  for  yellow  light  — 

On  cordierite  from  Ceylon,         a  =  1.537,    ft  =  1.542,    y  =  1.543 
"  "      Bodenmais,  a  =  1.535,    ft  =  1.541,    y  =  1.546 

"  "      Haddam,      a  =  1.5523,  ft  =  1.5615,  y  =  1.5627 

Hence  the  mean  index  of  refraction  is  about  the  same  as  that  of 
Canada  balsam,  y  —  a  =  0.008-0.009  ;  and  the  interference  colors  in 
thin  section  seldom  exceed  yellow  of  the  1st  order,  remaining  mostly 
in  the  gray-blue  and  white  tones  as  in  quartz.  The  axial  plane  lies  in 
GO  .P55  (100),  the  character  of  the  double  refraction  is  negative,  and  c  is 
the  acute  bisectrix  ;  hence  6  =  a,  J  =  C,  d  =  $  (Fig.  75).  The  apparent 
axial  angle  in  air  varies  within  wide  limits,  from  64°  (Haddam)  to  150° 
(Orijarfvi).  The  dispersion  is  weak,  p  <  v.  When  twinned,  basal 
sections  exhibit  the  axial  planes  in  convergent  light,  and  the  extinction 
in  parallel  light  inclined  60°  to  each  other  in  two  adjacent  individuals. 
For  sections  in  the  prism  zone  the  separate  individuals  extinguish  syn- 
chronously, whenever  the  twinning  plane  lies  parallel  or  perpendicular 
to  the  principal  section  of  the  polarizer  ;  or  else  the  twinning  may  be 
recognized  by  the  fact  that  the  different  individuals  exhibit  different 
interference  colors  during  a  rotation  of  the  section,  since  their  ellip- 


CORDIERITE. 


219 


soids  of  elasticity  are  intersected  differently.  The  pleochroism,  which 
is  generally  very  strong  in  thick  plates,  is  often  scarcely  noticeable  in 
thin  section,  yet  it  is  often  quite  strong  in  sections  from  the  prism 
The  facial  colors,  according  to  Haidinger,  are  oP  (001)  blue, 


zone. 


ooP56  (100)  bluish  white,  ooP<x>  (010)  yellowish  white.    The  following 
axial  colors  have  been  determined : 


LOCALITY.  a  b  c 

Bodenmais..  .light  Berlin-blue dark  Berlin-blue.. yellowish  white. 

Bodenmais..  .grayish  white milk-white yellowish  vinous,  yellow-white. 

Orijarfvi light  Berlin-blue dark  Berlin-blue.. reddish  clove-brown. 

Arendal plum-blue violet-blue reddish  clove-brown. 

Haddam bluish  white pale  blue yellowish  white. 

Simiulak dark  leather-brown,  .reddish  brown  to 

honey- yellow. . .  smoke-brown. 

The  absorption  b  >  a  >  c  may  even  be  noticed  in  colorless  sections., 
Pleochroic  halos  are  very  common  in  cordierite  ;  the  bright  yellow  halos 
surround  microscopic  inclusions  of  all  kinds;  the  color, is  a  maximum 
when  the  light  vibrates  parallel  to  c,  and_completely  disappears  when 
the  vibration  of  the  light  is  parallel  a  or  b.  Hence  basal  sections  da 
not  exhibit  this  phenomenon.  It  is  sometimes  observed  in  andalusiter 
staurolite,  augite,  muscovite,  etc.,  and  is  due  to  the  absorption  of  the 
blue  rays.  Hence  the  yellow  halo  appears  black  in  blue  light,  and 
does  not  appear  at  all  for  red  light.  Upon  being  heated  to  redness,, 
cordierite  loses  this  property  which  is  occasioned  by  a  local  accumula- 
tion of  an  organic  pigment.  It  has  only  been  observed  in  cordierite 
from  the  Archaean  and  from  contact  zones.  The  ordinary  pleochro- 
ism  of  cordierite  becomes  more  distinct  when  it  is  heated  to  redness, 
especially  in  thick  sections.  It  sometimes  disappears  from  thin  sec- 
tions upon  very  strong  heating. 

Sp.  gr.  =  2.59-2.66,  very  near  that  of  quartz,  so  that  it  is  often 
difficult  to  separate  the  two  mechanically.  Chemical  composition  = 
2MgO,  2A12O3,  5SiO2,  in  which  a  variable  amount  of  MgO  is  replaced 
by  FeO,  and  to  a  smaller  degree  by  MnO.  It  is  but  slightly  acted  on 
by  acids,  and  fuses  with  difficulty  on  the  edges.  The  chemical  distinc- 
tion from  quartz  is  furnished  most  simply  by  treating  the  section  with 
hydrofluosilicic  acid,  in  the  manner  already  described.  The  evapo- 
rated solution  yields  the  characteristic  prismatic  crystals  of  magnesium 
fluosilicate  (PL  XII.  Fig.  6).  The  surface  of  the  section  becomes 
covered  with  etched  figures,  whose  form  and  distribution  vary  with 
the  position  of  the  section.  In  sections  in  the  principal  zone  these 


220          PHYSIOGRAPHY   OF  THE  ROCK-MAKING  MINERALS. 

figures  have  the  form  of  long  rectangular  depressions.  Etched  figures 
are  also  produced  by  using  hot  sulphuric  acid,  which  is  a  mfeans  of 
distinction  from  quartz. 

Cordierite  has  no  constant  microstructure.  When  it  occurs  in 
eruptive  rocks  it  either  contains  fluid  and  glass  inclusions  alone,  as  in 
granites  and  quartz  porphyries,  or  it  is  almost  completely  free  from  in- 
terpositions, as  in  the  andesitic  rocks.  The  formation  of  cordierite  in 
these  rocks  belongs  to  an  older  period  of  rock  development,  and  ap- 
pears to  antedate  that  of  the  feldspars ;  for  many  of  these  occurrences 
its  nature,  as  a  normal  constituent,  is  very  doubtful,  and  its  derivation 
from  the  Archaean  rocks  through  which  the  eruptive  rocks  have  passed 
is  quite  probable.  The  real  home  of  cordierite  is  the  gneiss  formation, 
where  it  is  frequently  accompanied  by  garnet,  biotite,  sillimanite,  spinel, 
pyrrhotite,  hematite,  and  ilmenite.  All  these  minerals,  except  garnet, 
occur  as  inclusions  in  cordierite.  Besides  these  interpositions  fluid 
inclusions  are  common,  not  infrequently  with  colorless  cubes. 

Cordierite  is  present  in  many  schistose  hornstones  of  the  granite 
and  diorite  contact  zones,  frequently  accompanying  andalusite,  and  pos- 
sessing the  microstructure  of  the  cordierite  of  the  gneiss  formation,  but 
is  distinguished  from  this  by  the  frequency  of  normal  crystallographic 
boundaries. 

Cordierite  appears  to  be  readily  decomposed,  altering  to  more  or 
less  fibrous  or  lamellar  aggregates,  especially  in  the  gneiss  formations. 
These  decomposition  products,  or  mixtures  of  them  with  unaltered 
cordierite  substance,  are  variously  termed  aspasoli'te,  chlorophyllite, 
bonsdorffite,  esmarkite  (in  part),  pinite,  oosite,  praseolite,  gigantolite, 
fahlunite,  and  pyrargillite.  Many  of  these,  especially  pinite  and  oosite, 
consist  essentially  of  potash  rnica  in  an  irregular  intergrowth  of  lamel- 
lae ;  in  others  there  occurs,  besides  dense  muscovite,  a  chlorite  or  talc. 
The  dirty-brown  coloring  of  these  pseudomorphs  is  due  to  the  admix- 
ture of  limonite.  In  still  other  cases  there  result  yellowish  to  green- 
ish alteration  products,  which  do  not  permit  of  exact  determination, 
and  which  strikingly  suggest  the  serpentine  bands  of  olivine.  The 
process  of  decora positio*n  always  follows  the  cleavage  cracks  and  fissures 
of  the  mineral. 


ZOISITE. 


221 


Zoisite. 
Literature. 

FE.  BECKE,  Die  Gesteine  der  Halbinsel  Chalcidice.    T.  M.  P.  M.  1878.  I.  248-250. 
O.  LUEDECKE,  Der  Glaukophan  und  die  glaukophanfiihrenden  Gesteine  der  Insel 

Syra.     Z.  D.  G.  G.  1876.  XXVIII.  259-260. 
A.  SAUER,  Erlauterungen  zur  Section  Kupferberg  der  geologischen  Specialkarte  des 

Konigreichs  Sachsen.     Leipzig.  1882.  25. 
G.  TSCHERMAK  and  L.  SIPOCZ,  Beitrag  zur  Kenntniss  des  Zoisits.    S.  W.  A.  LXXXII. 

1880.  July. 

Zoisite  in  rocks  forms  either  isolated  crystals  or  prismatic  aggre- 
gates, consisting  of  parallel  or  slightly  divergent  columns.  Crystallo- 
graphic  boundaries  only  occur  in  the  vertical  zone;  the  faces  ooP 
(110),  with  116°  26',  predominate;  ooPoo  (010)  is  seldom  wanting: 
besides  these  there  is  often  a  great  number  of  derived  prisms  present, 
among  which  are  ooP4  (140),  o>P2  (120),  ooP2  (210),  and  ooP3 
(310).  Occasionally  when  terminal  faces  exist  they  appear  to  be  P 
(111)  and  2Poo  (021).  A  kind  of  hemimorphism  with  respect  to 
the  b  axis  is  quite  common,  the  prism  faces  being  developed  on  one 
side  of  the  crystal,  while  those  in  the  other  side  are  wanting,  being 
replaced  by  ooPoo .  Sections  of  the  crystals  at  right  angles  to  the 
prism  axis  are  rhombic  ( <*>P)?  or  apparently  hexagonal  (  ocP  .  oopoo  ), 
or  many-sided  to  round,  or  finally  triangular  to  trapezoidal ;  the  longi- 
tudinal sections  are  lath-shaped.  The  length  is  usually  three  times  the 
breadth,  or  more;  rarely  both  dimensions  are  approximately  equal,  and 
the  mineral  assumes  the  granular  form.  Twinning  cannot  be  detected 
morphologically,  but  from  the  optical  behavior  appears  to  be  quite  fre- 
quent. The  dimensions  vary  from  several  centimetres  to  microscopic 
proportions. 

Zoisite  is  characterized  by  a  very  perfect  cleavage  parallel  to 
ooPoo  (010),  which  is  shown  by  numerous  sharp  cracks  parallel  to  the 
longitudinal  direction  in  all  sections  in  the  prism  zone,  and  in  those 
parallel  to  the  base.  A  second  and  much  less  perfect  cleavage  runs 
parallel  to  ooPoo  (100),  and  is  seen  in  sections  parallel  to  ooPco 
(010),  and  parallel  to  the  base.  A  transverse  parting  approximately 
parallel  to  the  base  is  more  noticeable  the  longer  the  individuals.  The 
prisms  are  not  infrequently  bent.  The  base  appears  to  be  a  gliding- 
plane. 

When  fresh,  zoisite  is  transparent  and  colorless,  but  the  larger  crys- 
tals and  aggregates  are  often  clouded  peripherally,  the  smaller  ones 
completely  so  ;  they  then  appear  gray  to  greenish  gray.  Those  varie- 


222          PHYSIOGRAPHY  OF  THE  HOCK-MAKING  MINERALS. 

ties  colored  red  by  manganese  (thulite)  are  variously  colored  red,  yellow, 
or  almost  colorless  by  transmitted  light,  according  to  the  position  of  the 
section.  The  mean  index  of  refraction  is  quite  high,  fip  =  1.69-1.70 ; 
hence  the  relief  is  very  distinct,  and  the  surface  rough ;  on  the  other 
hand,  the  difference,  y  —  <*,  is  small  =  0.0054-0.0057,  consequently 
the  interference  colors  are  very  low.  In  very  thin  sections  not  parallel 
to  the  axial  plane  the  double  refraction  is  often  only  recognized  by 
using  sensitive  tones  of  color  (gypsum  or  quartz  plate).  ~No  other 
orthorhombic  mineral  has  so  little  double  refraction  with  so  high  an 
index  of  refraction.  Even  in  sections  parallel  to  the  axial  plane  the 
interference  colors  in  thin  section  only  exceed  yellow  of  the  1st  order 
when  the  section  is  quite  thick. 

The  axial  plane  in  some  cases  lies  in  the  principal  cleavage  face,  in 
others  in  the  basal  plane ;  in  fact  the  orientation  sometimes  varies  in 
one  and  the  same  crystal.  In  rock-making  zoisite  both  positions  ap- 
pear to  be  equally  common.  In  both  cases  a  is  the  acute  bisectrix,  and 
the  character  of  the  double  refraction  is  positive ;  in  the  first  instance 
the  optical  scheme  is  d  —  c,  T>  =  b,  6  =  d ;  in  the  second,  a  =  c,  b  =  a, 
•c  =  fc.  The  optic  angle  varies  between  wide  limits  from  almost  0°  to 
100°  in  air ;  it  is  usually  quite  small  in  rock-making  zoisite,  and  sections 
parallel  to  00^06  show  both  axes  even  in  air.  The  axial  figures  are  not 
infrequently  distorted.  The  strong  dispersion  is  characteristic ;  for  the 
basal  position  of  the  axial  plane  the  dispersion  is  always  p  >  v,  for  the 
brachypinacoidal  position  p  <  v . 

Colorless  zoisite  exhibits  no  pleochroism  and  no  absorption ;  but 
manganiferous  thulite  is  strongly  pleochroic.  Rays  vibrating  parallel 
to  b  are  yellowish,  those  parallel  to  H  rose-red,  those  parallel  to  a  red- 
dish white  in  very  thin  sections.  The  axial  plane  lies  in  the  plane  of 
the  yellow  and  reddish-white  rays,  the  dispersion  is  v  >  p ;  d  is  the 
positive  bisectrix. 

H.=  6-6.5.  Sp.  gr.  =  3.25-3.36.  Chemical  composition  =  H,O, 
4CaO,  3A12O3,  6SiO2.  Acids  do  not  attack  zoisite,  unless  it  has  been 
heated  to  redness.  It  fuses  with  intumescence,  and  then  gelatinizes 
with  HC1.  With  hydrofluosilicic  acid  the  powder  gives  a  strong  reac- 
tion for  calcium ;  in  the  solution  CsCl  produces  abundant  crystals 
of  caesium  alum. 

The  small  individuals  of  zoisite  are  generally  free  from  inclusions ; 
the  larger  ones  frequently  contain  fluid  inclusions,  either  round  or 
irregularly  shaped,  and  fine  tubular  canals  running  parallel  to  the 
cleavage,  and  occasionally  curving  and  branching  out  in  the  interior  of 
the  crystal.  Inclusions  of  amphibole  microlites  are  not  uncommon ; 


TALC. 


they  are  usually  arranged  with  their  longer  axis  parallel  to  the  vertical 
axis  of  the  zoisite. 

Zoisite  is  essentially  a  mineral  of  the  crystalline  schists,  especially 
of  the  hornblendic  members  (PL  XIX.  Fig.  4).  Its  appearance  as  an 
essential  constituent  of  the  so-called  saussurite  in  gabbros  is  an  alto- 
gether different  occurrence,  in  which  it  must  be  considered  as  the 
product  of  a  dynamo-metamorphic  alteration  of  plagioclase. 

Talc. 

Bock-making  talc  occurs  in  the  form  of  plates,  usually  elongated  in 
one  direction  like  flattened  rods ;  more  rarely  the  plates  are  developed 
equally  in  all  horizontal  directions,  and  hence  have  round  to  hexagonal 
(  ooP,  GO  Poo )  outlines.  The  tendency  to  a  rosette-like  arrangement 
is  to  be  noted,  leading  to  more  or  less  complete  spherulitic  forms;  an 
irregular  felting  of  the  plates  is  very  common.  The  perfect  cleavage 
parallel  to  oP  (001)  is  just  as  distinct  in  all  sections,  which  are  not  par- 
allel to  the  base,  as  in  the  micas. 

The  percussion  figure  (Schlag  figur\  a  six-rayed  star,  whose  rays 
intersect  at  60°,  one  of  them  being  perpendicular  to  oo.Pdo  (010),  in- 
dicates that  there  are  gliding  faces  in  the  prism  zone  ;  the  plates,  how- 
ever, occur  curved  and  bent  in  the  most  irregular  manner,  without 
breaks  or  cracks. 

Talc  is  transparent  and  colorless,  with  a  low  index  of  refraction,  which 
in  fact  is  not  directly  determinable,  but  is 
calculated  at  1.551.  The  double  refraction 
is  very  strong,  y  -  a  =  0.038-0.043.  The 
interference  colors,  therefore,  are  very  high, 
and  correspond  closely  to  those  of  musco- 
vite. 

The  axial  plane  is  in  the  macropinacoid, 
and  c  is  in  the  negative  bisectrix ;  the  axial 
angle  is  quite  small.  A  dispersion  is  not 
noticeable.  Fig.  76  gives  the  scheme  for 
talc  in  a  plate  bounded  by  the  prism  and  brachypinacoid.  The  dotted 
lines  indicate  the  percussion  figure,  the  third  ray  coinciding  with  5. 
Plates  parallel  to  oP  often  exhibit  a  slight  division  of  the  field  in  par- 
allel polarized  light,  and  the  interference  figure  is  variously  distorted 
in  convergent  light.  Sections  inclined  and  perpendicular  to  oP  ex- 
tinguish parallel  and  perpendicular  to  the  cleavage  lines.  The  rosette- 
like  and  spherulitic  aggregates  exhibit  in  parallel  light  between  crossed 


6=c 


224  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

nicols  the  dark  cross  parallel  to  the  principal  sections  of  the  nicols, 
with  the  light  quadrants  brightly  colored.  When  the  sections  are 
not  very  thin,  the  colors  are  whitish  red  and  whitish  green  of  the  4th 
order. 

H.  =  1.  Sp.  gr.  =  2.8.  Chemical  composition  =  3MgO,  4SiO2  + 
H2O.  Almost  infusible.  Heated  with  cobalt  solution  it  becomes  flesh- 
red.  Acids  are  almost  without  action  on  it. 

Talc  may  be  easily  confused  with  brucite  and  muscovite.  It  is  dis- 
tinguished from  the  first  by  its  optical  character  in  convergent  light, 
and  by  the  absence  of  an  alumina  reaction  in  the  fusion  with  alkali  car- 
bonates ;  from  the  second  by  the  absence  of  alkalies  when  treated  with 
hydrofluosilicic  action  and  by  the  reaction  with  cobalt  solution. 

Talc  is  usually  free  from  inclusions,  but  the  larger  lamellae  are  often 
pierced  by  tremolite  needles,  and  frequently  enclose  biotite. 

Talc  is  found  as  an  essential  rock  constituent  in  the  region  of  the 
crystalline  and  dynamo-metamorphic  schists.  It  is  often  accompanied 
by  rhombohedral  carbonates  and  by  quartz.  In  eruptive  rocks  it  only 
occurs  as  pseudornorphs  after  magnesian  silicates,  and  rarely  then. 
G.  H.  Williams*  has  observed  its  occurrence  as  an  alteration  product 
of  hornblende  in  the  magnesian  rocks  from  the  vicinity  of  Baltimore, 
Md. 

Natrolite. 

Natrolite  is  never  a  primary  constituent  of  rocks,  but  it  is  extremely 
common  as  an  alteration  product  of  sodalite,  nosean,  nepheline  and 
acid  plagioclase,  partly  in  actual  pseudornorphs  after  these  minerals, 
partly  in  cavities  and  cracks.  It  almost  always  forms  radial  aggregates, 
which  are  sometimes  parallelly  fibrous,  sometimes  divergent  to  radially 
fibrous,  not  infrequently  they  form  spherulites.  The  long  axis  of  the 
individuals  corresponds  to  the  vertical  crystallographic  axis. 

Cleavage  is  seldom  observed  on  the  prismatic  masses  ;  when  present 
it  is  parallel  to  the  prism,  and  in  cross-sections  appears  as  a  system  of 
cracks  intersecting  apparently  at  right  angles.  It  is  colorless  by 
transmitted  light,  if  the  separate  individuals  are  not  too  minute,  or 
yellowish  to  brownish  and  cloudy  for  very  small  transverse  dimen- 
sions. The  coefficient  of  refraction  is  small,  the  double  refraction 
is  measurable.  <xp  =  1.4768,  /?p  —  1.4797,  yp  =  1.4887.  The  axial 
plane  lies  in  &Pv5  (010),  and  the  vertical  axis  is  the  positive  acute  bi- 
sectrix ;  hence  a  —  a,  I  —  b,  c  —  c.  The  axial  angle  in  air  is  over  90°. 

*  Bulletin  28,  U.  S.  Geol.  Survey,  1886,  p.  58. 


NATROLITE.  225 

Dispersion  p  <  v.  The  extinction  in  longitudinal  sections  is  parallel 
to  the  axis  of  the  prism,  in  cross-section  it  is  diagonal  to  the  cleavage. 
The  radially  columnar  aggregates  give  neat  interference  crosses  in  par- 
allel light  between  crossed  nicols,  the  arms  lying  parallel  to  the  prin, 
cipal  sections  of  the  nicols. 

H.  =  5-5.5.  Sp.  gr.  =  2.17-2.26.  Chemical  composition  = 
NaQO,  A12OS  -|-  2aq.  Gelatinizes  easily  with  hydrochloric  acid.  The 
aggregates  usually  show  very  pure  substance  ;  they  occasionally  enclose 
plates  of  micaceous  iron  and  interpositions  of  the  parent  minerals. 

Appendix. — A  corresponding  lime  zeolite,  probably  scolecite,  often 
occurs  as  an  alteration  product  of  the  more  basic  feldspars,  (cf.  Kloos, 
N.  J.  B.  G.  H.  Williams,  Bulletin  28,  U.  S.  Geological  Survey.) 


Dumortierite. 

The  translator  is  indebted  to  Mr.  J.  S.  Diller  for  the  following 
notes  on  the  occurrence  of  dumortierite  in  the  United  States. 

Dumortierite  occurs  in  columnar  to  fibrous  masses  associated  with 
tourmaline  in  certain  Archaean  rocks.  "When  crystal  form  is  observed, 
the  planes  oojPoo  (100)  and  oojP  (110)  are  equally  developed,  the  angle 
between  them  being  about  152°.  Terminal  planes  rarely  visible  on 
imbedded  crystals.  Cleavage  parallel  to  <x>Pao  seldom  observed  in 
thin  section,  but  developed  by  pressure  together  with  one  parallel  to  a 
prismatic  plane.  Poly  synthetic  twinning  frequent. 

Its  index  of  refraction  is  higher  than  that  of  quartz ;  double  refrac- 
tion rather  strong,  interference  colors  those  of  the  2d  order.  Optical 
character  negative ;  acute  bisectrix  parallel  to  the  vertical  crystallo- 
graphic  axis.  It  is  transparent  blue  with  highly  characteristic 
pleochroism ;  the  extraordinary  ray  is  deep  cobalt  blue,  the  ordinary 
ray  is  colorless.  It  loses  its  color  upon  being  heated  to  redness. 

H.  =  7.  Sp.  gr.  =  3.265.  Chemical  composition  not  yet  definitely 
determined,  a  silicate  of  alumina.  Insoluble  in  acids,  including  hydro- 
fluoric. 

Dumortierite  occurs  chiefly  in  quartz,  sometimes  in  hair-like  forms, 
in  the  peginatitic  portion  of  a  biotite  gneiss  at  Harlem,  New  York,  a 
rock  similar  to  that  near  Lyons  in  which  it  was  originally  discovered 
by  Gounard  (Bui.  Soc.  Min.  Fr.  Vol.  IV.  p.  2.  1881).  It  also  occurs 
in  granular  quartz  near  Clip,  Arizona. 


226         PHYSIOGRAPHY  OF  THE  ROCK-MAKIflG  MINERALS. 


MINERALS  OF  THE  MONOCLINIC  SYSTEM. 

SECTIONS  of  a  regularly  developed  crystal  of  the  monoclinic  system 
and  the  figures  made  by  the  cleavage  cracks  are  only  symmetrical  when 
they  belong  to  the  orthodiagonal  zone.  The  cleavage  is  either  single 
(parallel  to  pinacoids  or  orthodomes),  and  lies  in  the  plane  of  symmetry 
or  stands  at  right  angles  to  it ;  or  the  cleavage  is  the  same  parallel  to  two 
faces  (prismatic)  which  make  equal  angles  with  the  plane  of  symmetry. 
A  single  cleavage  gives  a  single  system  of  parallel  cleavage  cracks  in 
all  sections  but  those  parallel  to  the  cleavage  face.  Two  single  cleav- 
ages occurring  in  the  same  crystal  cannot  be  equal ;  they  furnish  par- 
allel cleavage  cracks  in  all  sections  in  the  zone  of  the  cleavages,  and 
intersecting  systems  of  dissimilar  cleavage  cracks  in  all  other  sections. 

The  ellipsoid  of  elasticity  in  monoclinic  crystals  is  triaxial ;  one  axis 
coincides  with  the  orthodiagonal  or  axis  of  symmetry  of  the  crystal ; 
the  two  other  axes  lie  in  the  plane  of  symmetry,  ooPob  (010).  If  the 
axis  of  elasticity  which  coincides  with  the  orthodiagonal  is  b,  then  the 
optic  axes  lie  in  the  plane  of  symmetry  (symmetrical  axial  position), 
and  are  dispersed  in  this  plane  together  with  the  bisectrices  (inclined 
dispersion)  ;  if  one  of  the  bisectrices  coincides  with  the  orthodiagonal, 
then  the  optic  axes  lie  in  a  plane  of  the  orthodiagonal  zone  (normal 
symmetrical  axial  position).  The  dispersion  then  is  either  horizontal 
or  crossed,  according  as  the  orthodiagonal  is  the  obtuse  or  acute  bisec- 
trix. Therefore  all  sections  in  the  orthodiagonal  zone  during  a  com- 
plete revolution  between  crossed  nicols  extinguish  light  four  times, 
parallel  and  perpendicular  to  the  single  cleavages  diagonal  to  the  pris- 
matic cleavage  (parallel  extinction) ;  all  other  sections  which  are  not 
perpendicular  to  an  optic  axis,  during  a  complete  revolution  extin- 
guish four  times  in  positions  inclined  at  a  certain  angle  to  the  principal 
sections  of  the  nicols  (inclined  extinction).  The  inclination  of  the 
axes  of  elasticity  lying  in  the  plane  of  symmetry  to  the  crystal  axes  is 
called  the  extinction  angle,  and  is  an  important  means  of  distinguishing 
monoclinic  minerals.  Sections  at  right  angles  to  an  optic  axis  remain 
uniformly  light  during  a  rotation  between  crossed  nicols.  In  converg- 
ent light  sections  perpendicular  to  an  optic  axis  or  not  much  inclined 
to  it,  as  well  as  those  perpendicular  to  a  bisectrix,  exhibit  the  same  in- 
terference figures  as  similarly  situated  sections  in  orthorhombic  crystals. 


GYPSUM.  227 

But  in  sections  in  the  first  position  the  axial  bar  has  differently  colored 
borders  if  the  substance  has  sufficiently  strong  dispersion  and  the  axial 
plane  is  perpendicular  to  the  plane  of  symmetry;  and  in  sections  in  the 
second  position  the  distribution  of  the  colors  is  not  bisymmetrical  as  in 
the  orthorhombic  system,  but  is  monosymmetric  with  respect  to  the 
plane  of  symmetry  if  the  bisectrix  lies  in  this  plane  (inclined  and  hori- 
zontal dispersion),  and  symmetrical  with  respect  to  the  centre  of  the 
interference  figure  if  the  orthodiagonal  is  the  bisectrix  (crossed  dis- 
persion). The  distribution  of  blue  and  red  in  the  innermost  color  rings 
or  on  the  poles  of  the  hyperbolas  in  the  diagonal  position  determines 
in  this  system,  as  in  the  orthorhombic  system,  the  relative  size  of  the 
angles  of  the  optic  axes,  p  >  v  or  p  <  v. 

If  monoclinic  minerals  exhibit  pleochroism,  all  sections  are  dichroic 
which  are  not  perpendicular  to  an  optic  axis ;  the  maximum  differences 
of  color  lie  90°  from  one  another,  and  necessarily  coincide  with  the  di- 
rections of  extinction  in  sections  in  the  orthodiagonal  zone ;  this  coin- 
cidence generally  exists  in  all  other  sections,  but  not  necessarily. 

Many  monoclinic  minerals  (mica)  in  their  optical  characters  strik- 
ingly approach  those  of  the  hexagonal  or  orthorhombic  system,  and 
with  the  ordinary  microscopical  investigation  are  only  recognized  as 
monoclinic  with  great  difficulty  or  not  at  all. 


Gypsum. 

Literature. 

En.   HAMMERSCHMIDT,   Beitrage  zur  Kenntniss  des  Gyps-  und  Anhydritgesteins. 
T.  M.  P.  M.  1882.  V.  245-285. 

Rock-making  gypsum  shows  no  crystallographic  boundaries  ;  it 
appears  in  irregular  granular  aggregates.  In  secondary,  possibly  pri- 
mary, veins  which  traverse  the  granular  masses  it  is  lamellar  to  fibrous, 
the  fibres  standing  perpendicular  to  the  walls  of  the  veins.  The  sec- 
tions therefore  exhibit  no  characteristic  forms. 

The  perfect  cleavage  parallel  to  oo.P6b  (010)  gives  rise  to  abundant 
parallel  cracks.  The  fibrous  fracture  as  well  as  the  conchoidal  fracture 
and  gliding  planes  parallel  to  Poo  (101).  and  |Poo  (509)  are  seldom 
observed  microscopically  in  rocks. 

Gypsum  becomes  transparent  and  colorless  or  is  gray  to  grayish 
blue  from  carbonaceous  matter,  and  reddish  to  yellowish  from  hydrous 
oxide  of  iron,  or  from  plates  of  ferric  oxide.  The  index  of  refraction 
is  small;  the  double  refraction  measurable,  ana  —  1.5207,  /3na  =  1.5228, 


228         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

y^  —  1.5305.  The  axial  plane  lies  in  the  plane  of  symmetry;  the  in- 
clined dispersion  is  distinct.  2  Y=  61°  24/  becomes  rapidly  smaller 
with  increase  of  temperature.  The  acute  bisectrix  lies  in  the  obtuse 
angle  between  the  vertical  and  clinodiagonal  axes,  and  is  inclined 
75°  15'  to  the  former  and  23°  42'  to  the  latter. 

II.  =  2.  Sp.gr.  =  2.2-2.4.  Chemical  composition  =  CaO,  SO,  -[- 
2aq.  Difficultly  soluble  in  water.  Gives  off  much  water  in  a  closed 
tube,  and  fused  with  soda  on  charcoal  gives  the  reaction  for  sulphur. 

Not  infrequently  gypsum  encloses,  besides  carbonaceous  substances 
and  iron  oxides,  fluid  and  gas  inclusions  of  irregular  shape  or  in  nega- 
tive crystals.  Calcite,  magnesite,  dolomite,  and  quartz  are  found  in  it 
in  crystals  or  grains. 

Gypsum  only  occurs  in  the  gypsum  rock  and  in  the  anhydrite  of 
sedimentary  formations. 

Wollastonite. 

Literature. 

A.  E.  TORNEBOHM,  Nefelinsyenit  fi&n  Alno.     Geol.    Foren.  i  Stockholm  ForhdL 
1883.  VI.  No.  82.  543-549. 

Wollastonite  appears  as  incompletely  bounded  prismatic  or  tabular 
crystals,  which  are  always  elongated  parallel  to  the  axis  of  symmetry 
(6),  or  they  are  in  prismatic  or  fibrous  aggregates  with  a  more  or  less 
parallel  or  slightly  divergent  arrangement  of  the  individuals.  There- 
fore, sections  of  the  isolated  individuals  from  the  orthodiagonal  zone 
are  lath-shaped,  cross-sections  are  round  to  six  or  eight  sided  from  the 
faces  oP  (001),  ooPcfe  (100),  £P<55  (102),  and  -  Pk  (101).  The  angles 
are  001  AlOO  =  95°  30',  100 AlOl  =  44°  27',  100 A102  =  69°  56'.  The 
face  oo  jPoo  (100)  is  usually  the  most  broadly  developed.  Twinning  is 
quite  frequent  according  to  the  law  :  the  twinning  plane  is  oo  P  55  (100); 
the  faces  oP  of  the  individuals  making  an  angle  of  169°  with  one 
another. 

The  cleavage  is  perfect  parallel  to  o  P  (001)  and  ooT^ob  (100),  less 
perfect  parallel  to  £Poo  (102)  and  —  P55  (101).  The  last-named  face 
is  inclined  50°  25'  to  ooPo5  (100).  In  sections  from  the  orthodiago- 
nal zone  all  the  cleavage  cracks  are  parallel  to  one  another ;  in  sections 
parallel  to  ccPoo  they  form  two  very  distinct  systems  of  cracks  inter- 
secting at  84°  30',  which  are  sometimes  cut  diagonally  by  two  other 
systems  which  are  neither  so  distinct  nor  so  numerous.  In  the  ortho- 
diagonal  sections  the  cleavages  run  parallel  to  a  longitudinal  fibration, 
usually  quite  distinct,  which  is  due  to  the  parallel  growth  of  very 


WOLLASTONITE. 


229 


slender  individuals.     Larger,  irregular  cracks  stand  at  right  angles  to 
the  length  of  these  sections. 

Wollastonite  becomes  transparent  and  co/orless,  and  possesses  a 
mean  index  of  refraction  which  is  not  inconsiderable  =  1.635.  The  in- 
terference colors  are  quite  bright  parallel  to  the  plane  of  the  optic 
axes  ;  y  —  a  =  0.016.  The  axial  plane  lies  in  the  clinopinacoid ;  2^,  = 
70°  40';  ZEV  =  68°  24'.  The  positive  acute  bisectrix  lies  in  the  ob- 
tuse angle  /?,  and  makes  an  angle  of  about  37°  40'  with  the  cleavage 
parallel  to  ccPcc  (100).  The  inclined  dispersion  shows  itself  by  a  lively 
difference  of  color  on  the  margin  of  the  hyperbola  of  one  axis  (red 
inside,  blue  outside),  while  the  colors  of  the  second  hyperbola  are 
blue  inside  and  outside.  Fig.  77  presents  the  optical  scheme  for  the 
plane  of  symmetry.  It  is  seen  that  an  axis  stands  perpendicular  or 
only  slightly  inclined  to  each  principal  face.  Twinning  parallel  to 
coP56  (100)  can  be  recognized  by  the  fact  that  in  all  sections  inclined 
to  the  twinning  plane  the  lamellae  do 
not  extinguish  at  the  same  time;  in 
sections  from  the  orthodiagonal  zone, 
-although  the  lamellse  extinguish  to- 
gether, they  can  be  recognized  by  their 
different  interference  colors,  which  are 
due  to  the  fact  that  the  ellipsoid  of 
elasticity  is  cut  differently  in  each  half 
of  the  twin  and  o  —  e  is  different  in 
each  case.  In  convergent  light  sections 

£j  O 

in  the  orthodiagonal  zone  exhibit  axial 

iigures,  and  the  points  of  emergence  of 

bisectrices,  and  the  axial  plane  always 

lies  perpendicular    to    the    longitudinal    direction    and    the   cleavage. 

There  is  no  plaochroism. 

H.  —  4.5-5.0.  Sp.  gr.  =  2.8-2.9.  Chemical  composition  =  CaO, 
SiO2.  It  gelatinizes  easily  with  hot  hydrochloric  acid ;  there  is  an 
abundant  reaction  for  gypsum  upon  adding  sulphuric  acid  to  the 
solution  :  anhydrite  forms  in  a  very  concentrated  solution,  the  crystals 
being  rhombic  with  a  cubical  habit.  The  powder  fuses  with  great 
difficulty.  Its  gelatinization  with  HC1  is  an  important  means  of  dis- 
tinguishing it  from  epidote,  with  the  colorless  varieties  of  which  it 
may  be  confused,  because  of  the  similar  prismatic  development  parallel 
to  b  and  of  the  same  position  of  the  axial  plane  with  reference  to  the 
longitudinal  axis,  in  spite  of  the  high  index  of  refraction  and  higher 
double  refraction  of  the  latter. 


230         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

Wollastonite  has  no  constant  microstructure  :  it  often  contains  fluid 
inclusions,  grains  of  calcite  and  diopside,  or  other  minerals  associated 
with  it. 

Wollastonite  is  a  frequent  guest  in  granular  limestone  and  in  the 
rocks  related  to  it  occurring  in  the  Archaean  (garnet  rock,  epidote 
rock,  etc.),  and  it  not  infrequently  occurs  in  feldspathic  schists  when 
these  are  rich  in  lime.  It  is  also  found  in  contact-metamorphosed 
limestones  and  in  limestone  inclusions  in  eruptive  rocks,  where  it  is 
usually  accompanied  by  pyroxene  and  garnet,  as  in  the  schists.  It  is 
very  rarely  found  in  eruptive  rocks. 

Group  of  Monoclinic  Pyroxenes. 

Literature. 
P.  MANN,  Untersuchungen  tlber  die  chemische  Zusammensetzung  der  Augite  aus1 

Phonolithen  und  verwandten  Gesteinen.     K  J.  B.     B.-B.  II.  1884.  173-205. 
A.  MERIAN,  Studien  an  gesteinsbildenden  Pyroxenen.    N.  J.  B.    B.-B.  III.  1884. 

252-315. 
A.  MICHEL-LEVY,  De  I'emploi  du  microscope  polarisant  &  lumiere  parallele  pour 

1'etude  des  plaques  minces  de  roches  eruptives.    Ann.  des  Mines.  Paris.  1877. 

(7).  XII.  424-429. 

G.  TSCHEBMAK,  Ucber  Pyroxen  und  Amphibol.     T.  M.  M.  1871.  17. 
—  Mikroskopische  Unterscheidung  der  Mineralien  aus  der  Augit-,  Amphibol-  und 

Biotitgruppe.    S.  W.  A.  1.  Abth.  1869.  LIX.  May. 

Only  those  minerals  of  the  monoclinic  system  commonly  referred 
to  the  pyroxene  family  are  here  grouped  as  monoclinic  pyroxenes,, 
in  which  the  characteristic  cleavage  parallel  to  an  almost  right- 
angled  prism  is  distinctly  noticeable. 

The  monoclinic  pyroxenes  belong  to  the  most  widely  distrib- 
uted rock-making  minerals,  both  in  eruptive  rocks  and  in  the  crystal- 
line schists ;  they  appear  in  perfectly  developed  crystals,  in  irregularly 
bounded  individuals,  or  in  aggregates.  The  habit  varies  with  the 
chemical  composition.  It  may  be  stated  as  the  rule, — which,  however, 
is  not  without  exceptions, — that  pyroxenes  of  the  diopside  and  acmite 
series  usually  form  long  columnar  crystals  with  highly  subordinate 
prism  faces,  and  columnar  masses ;  pyroxenes  of  the  augite  series 
form  short  prismatic_  individuals  and  grains.  The  commonest  crystal 
forms  based  on  a :  b  :  c  =  1.0903 : 1 :  0.5893  and  ft  —  74°  II7  are  m  = 
ooP  (110)  with  87°  06',  a  =  ooP<£  (100),  ~b  —  ooPob  (010),  s  =  P  (111) 
with  111  A  111  =  120°_48',j*  =—P  (111)  with  111  A  111  =  131°  30', 
o  =  2P  (221)  with  221 A221  =  95°  48',  p=  P^>  (101),  c  =  oP  (001), 
n  =  £P56  (102)  with  102  A 100  =  89°  38'.  Fig.  78  shows  one  of  the 
most  frequent  forms  of  rock-making  diopside ;  Fig.  79  such  a  one  of 


MONOCLIN1C  PYROXENES. 


231 


nugite.  Hence  cross-sections  more  or  less  perpendicular  to  c  exhibit 
squares  with  slightly  truncated  corners,  or  octagons  with  sides  of 
almost  equal  length ;  sections  from  the  orthodiagonal  zone  give  lath- 
shaped  figures,  either  pointed  quite  steeply  or  cut  off  straight,  some- 
times almost  hexagonal ;  sections  lying  more  or  less  parallel  to  the  plane 
of  symmetry  are  lath-shaped,  with  one  or  two-sided  terminations  or 
slightly  prolonged  inclined  rhombs.  Sections  through  irregularly 
bounded  individuals  may  be  of  almost  any  shape. 

Twinning  is  extremely  common,  and  usually  follows  the  law :  the 
twinning  plane  is  oo  .Poo  (100).  In  the  diopsides  and  acmites  the 
twinning  line  very  frequently  runs  through  the  middle  of  the  crystal ; 
hence  the  outline  shows  no  re-entrant  angle  and  the  twinning  is  only 
recognized  between  crossed  nicols.  Fig.  80  shows  the  form  which 


m  a 


m 


Fife.  78 


Kig. 


.  SO 


arises  in  augites  ;  hence  the  twinning  is  not  noticeable  in  the  outline 
of  sections  in  the  orthodiagonal  zone,  but  it  is  in  that  of  sections  in 
the  prism  and  clinodiagonal  zone.  Between  the  larger  halves  of  twin 
crystals  there  often  appear  a  number  of  smaller  twinned  lamellae 
(PL  XIX.  Fig.  5).  A  second  twinning,  occurring  especially  in  the 
diopsides  and  diallages,  follows  the  law  :  the  twinning  plane  is  the 
base.  In  this  case  the  form  of  development  is  usually  lamellar,  a 
number  of  twinned  lamellae  being  enclosed  in  a  larger  individual. 
This  is  not  noticeable  in  the  outer  contours,  but  is  detected  on  the 
vertical  faces  of  the  crystal  as  a  fine  striation  at  right  angles  to  the 
axis  of  the  prism;  in  sections  it  is  generally  noticeable  even  in  ordi- 
nary light,  and  comes  out  distinctly  between  crossed  nicols  (PL  XIX. 
Fig.  6).  Both  of  these  kinds  of  twinning  occur  in  the  same  crystal  in 
the  diallage-like  augites  of  many  diabases.  The  twinnings  parallel  to 
-  Po5  (101)  (Fig.  81)  and  parallel  to  P2  (122)  (Fig.  82)  are  rarer,  and 
are  principally  confined  to  basaltic  augite.  They  generally  appear  in 


232 


PHYSIOGRAPHY  OF  THE  HOCK-MAKING  MINERALS. 


the  form  of  complicated  intergrowths  of  several  augite  individuals 
(PL  XX.  Fig.  1). 

The  dimensions  of  pyroxene  crystals  vary  greatly ;  in  the  eruptive 
rocks  especially  they  sink  to  microlitic  proportions.  Here  also  occur 
the  greatest  variety  of  imperfect  crystal  forms ;  not  infrequently  these 
incipient  forms  of  growth  are  found  in  larger  individuals  (PI.  XX. 
Fig.  2).  The  incomplete  development  is  generally  confined  to  the 
terminal  faces.  Skeleton  crystals  also  (PI.  III.  Fig.  3),  whose  arms 
intersect  at  pyroxene  angles,  are  not  uncommon,  besides  extremely 
delicate  and  capricious  forms  of  growth,  at  times  approaching  spheru- 
litic  forms  (PL  XX.  Fig.  3);  these,  however,  are  confined  to  the 
glassy  eruptive  rocks. 

Shelly  structure  is  frequent  in  augite  and  acmite,  and  from  the 
variety  of  chemical  composition  appears  in  the  form  of  isomorphous 


ITig..  81 


.  82 


layers  or  as  zonal  structure.  Generally  these  successive  shells  are 
geometrically  similar  to  one  another  and  to  the  outward  form  of  the 
crystal,  but  occasionally  the  inner  shells  exhibit  a  different  crystnllo- 
graphic  outline  from  the  outermost  shell  (PL  XX.  Fig.  4).  The 
number  of  shells  varies  greatly.  Moreover,  the  outline  of  the  inner 
shells  is  not  always  a  crystallographic  one  :  sometimes  they  merge  into 
one  another  (PL  XX.  Fig.  2),  or  it  is  evident  that  the  kernel  was  at 
one  time  a  corroded  crystal  (PL  XX.  Fig.  4).  Quite  rarely  the 
shelly  structure  follows  oP  (001),  when  it  is  accompanied  by  the 
twinning  and  parting  parallel  to  this  face.  Hour-glass  forms  (PL  Y. 
Fig.  6)  are  produced  by  the  filling  up  of  the  gaps  of  forked  crystals 
by  newer  pyroxene  substance. 


MONOCLINIC  PYROXENES.  233 

Corrosion  phenomena  are  not  infrequent,  especially  on  the  older 
pyroxenes  of  the  eruptive  rocks ;  mechanical  deformations  in  the 
shape  of  bendings,  breakings,  shatterings,  and  tortioiis  occasionally 
•occur  in  the  pyroxenes  of  all  rocks,  but  are  quite  rare  in  those  of  the 
Archaean  rocks,  because  the  pyroxenes  appear  to  be  unable  to  with- 
stand the  mechanical  processes  which  these  undergo ;  they  are  here 
converted  into  amphibole. 

The  monoclinic  pyroxenes  cleave  with  variable  perfection  parallel 
to  the  prism  of  87°  06'.  The  cracks  corresponding  to  this  cleavage 
are  almost  always  distinct  and  numerous,  but  they  seldom  run  unin- 
terruptedly and  straight  through  the  entire  crystal.  In  sections  ap- 
proximately perpendicular  to  the  prism  axis  they  form  two  systems, 
crossing  each  other  nearly  at  right  angles  (PL  X.  Fig.  4) ;  in  sections 
in  the  prism  zone  they  run  parallel ;  in  all  other  sections  they  make 
rhombic  figures  whose  angles  depend  on  the  position  of  the  section 
with  respect  to  the  crystal  (PL  XX.  Fig.  5).  Besides  the  prismatic 
cleavage  there  is  also  a  cleavage  parallel  to  one  or  both  vertical  pina- 
coids,  especially  in  diallage  and  diopside,  less  frequently  in  the  augites 
and  acmites ;  it  is  always  quite  imperfect,  and  is  only  indicated  by 
short  or  intermittent  cracks.  Individuals  of  the  diopside  and  diallage 
series  twinned  parallel  to  oP  (001)  exhibit  quite  a  perfect  parting  paral- 
lel to  this  face  (PL  XIX.  Fig.  6),  which,  however,  does  not  represent 
a  proper  cohesion  minimum,  but  is  due  to  the  twin  lamination.  In 
some  basaltic  rocks  there  are  augites  which  exhibit  neither  macro- 
scopic nor  microscopic  cleavage.  All  monoclinic  pyroxenes  when 
in  long  prismatic  forms  exhibit  an  irregular  parting  approximately 
perpendicular  to  the  prism  axis. 

All  monoclinic  pyroxenes,  even  when  strongly  colored,  become  per- 
fectly transparent ;  except  the  acmites,  which  are  not  very  transparent. 
The  colors  in  transmitted  light  are  very  different  according  to  the  chem- 
ical composition,  and  therefore  change  in  one  and  the  same  crystal  with 
the  isomorphous  layers.  The  diopsides  and  diallages  are  mostly  quite 
colorless  to  light  greenish  ;  the  latter  are  also  brown  ;  the  augites  and 
acmites  are  green  or  brown  to  violet,  in  different  shades.  A  deep 
brownish-red  to  brownish-violet  color  seems  to  indicate  a  not  incon- 
siderable percentage  of  titanium.  Yellowish  augites  are  rarer,  and 
are  almost  exclusively  confined  to  certain  trachytic  and  andesitic  rocks. 
A  red  color  only  occurs  secondarily  in  augites  which  have  been  heated 
to  redness,  and  may  be  produced  artificially  in  this  way  from  green 
augites.  The  pyroxenes  of  the  acid  and  alkali  rocks  are  predomi- 
nantly green  ;  those  of  basic  eruptive  rocks  and  such  as  are  poor  in. 


234          PHYSIOGRAPHY  OF  THE  HOCK-MAKING  MINERALS. 


alkali  are  brown.     The  monoclinic  pyroxenes  of  the  schists  are  usually 
colorless  or  greenish. 

All  monoclinic  pyroxenes  have  the  optic  axes  in  the  plane  of  sym- 
metry ;  they  all  possess  a  high  index  of  refraction  and  strong  positive 
double  refraction.  But  the  angle  of  the  optic  axes  and  the  inclination 
of  the  bisectrices  vary  considerably,  and  in  the  general  remarks  on  the 
optical  orientation  acmite  must  be  omitted. 

Des  Cloizeaux  found  in  the  clear  diopside  from  Ala  for  yellow 
light  a  =  1.6727,  ft  =  1.6798,  y  =  1.7062.  Hauser  found  for  the 
same  occurrence,  /3na  —  1.68135.  Tchihatcheff,  in  diopside  from  Ziller- 
tlial,  /3na  =  1.67996.  A.  Schmidt,  in  diopside  from  Ducktown,  Ten- 
nessee, fina= 1.6902.  Tscherrnak,  in  coccolite  from  Arendal,  ftp  =1.690; 
in  dark-green  diopside  from  Nordmarken,  fip  =  1.701 ;  in  augite  from 
Borislau,  /?=1.70;  and  in  that  from  Frascati.  /?=:1.74,  approximately, 
These  figures  explain  the  strong  relief  and  the  rough  surface  of  mono- 
clinic  pyroxenes.  The  difference  y  —  a  =  0.0335  determines  the  bright 
interference  colors;  a  section  parallel  to  ooPob  (010)  of  only  0.02  mm. 

thickness  gives  colors  of  the  2d  order  ;, 
this  strong  double  refraction  is  an  im- 
i  portant  means  of  distinguishing  them 
from  orthorhombic  pyroxenes.  For  all 
monoclinic  pyroxenes,  except  acmite,, 
the  positive  acute  bisectrix  lies  in  the 
obtuse  axial  angle,  and  forms  a  variable 
angle  with  the  vertical  crystal  axisr 
which,  however,  is  always  large.  Hence 
sections  parallel  to  the  plane  of  sym- 
metry show  the  maximum  of  darkness 
between  crossed  nicols  when  the  pris- 
matic cleavage  is  highly  inclined  to  the 
principal  sections  of  the  nicols,  and 
this  large  extinction  angle  (between  36° 
30'  and  54°)  is  one  of  the  most  char- 
acteristic properties  of  the  monoclinic 
The  scheme  (Fig.  83)  gives  the  optical  orientation  in  a 
diopside  poor  in  iron,  and  exhibits  the  position  of  the  optic  axes  and 
bisectrices  in  the  plane  of  symmetry.  The  extinction  angle  varies 
with  the  chemical  composition  of  the  pyroxene,  but  in  exactly  what 
ratio  is  not  yet  definitely  known.  It  is  least  in  the  diopsides  and 
diallages  poor  in  alumina  and  iron  ;  in  these  the  extinction  angle  c  A  c 
lies  between  36°  and  40°  ;  it  increases  with  the  percentage  of  iron  and 


Fig.  83 


pyroxenes. 


MONOCLINIC  PYROXENES.  235 

alumina;  in  the  angites  proper  it  varies  from  41°  to  54°,  lying  mostly 
between  43°  and  48°.  It  naturally  varies  in  the  different  isomorphous- 
shells  of  zonally  built  crystals. 

The  behavior  of  sections  in  the  three  principal  zones  in  parallel 
polarized  light  between  crossed  nicols  is  evident  from  the  foregoing. 
In  sections  from  the  zone  oP  :  &Pa5  (001 :  100)  the  cleavage  cracks 
form  rhombic  figures  whose  anterior  angle  of  84°  49'  first  increases  to 
87°  06',  then  decreases  to  0°  ;  while  the  side  angle  decreases  from  95° 
IT  to  92°  54',  and  then  increases  to  180°.  The  extinction  is  always 
symmetrical  to  the  cleavage  cracks ;  it  bisects  their  angle  as  long  as- 
they  intersect  one  another,  and  lies  parallel  to  them  when  they  are 
parallel  to  each  other.  This  is  the  behavior  of  all  monoclinic  minerals 
when  the  zonal  axis  coincides  with  an  axis  of  the  ellipsoid  of  elas- 
ticity. Sections  from  the  zone  ccPvo  :  <x>Po5  (010  : 100)  are  recognized 
by  the  fact  that  the  cleavage  cracks  always  run  parallel ;  the  extinction 
angle  has  its  maximum  in  the  plane  of  symmetry,  and  decreases 
steadily  to  0°  in  sections  parallel  to  ooPoo  (100).  In  sections  from  the 
zone  oP :  ooPoo  (001  :  010)  the  cleavage  cracks  form  rhombic  figures 
whose  anterior  angle  decreases  from  84°  49'  to  0°.  In  the  section  par- 
allel to  oP  (001)  the  extinction  is  sj-mmetrical  to  the  cleavage  cracks, 
then  rapidly  becomes  quite  inclined,  reaches  a  maximum  which  is 
slightly  greater  than  the  extinction  angle  on  &>Pcc  (010),  and  then 
falls  slowly  to  the  angle  corresponding  to  this  face  (Fig.  83). 

The  angle  between  the  optic  axes  of  monoclinic  pyroxenes  varies 
with  the  chemical  composition  just  as  the  extinction  angle  does;  in 
general,  it  appears  to  be  smaller  as  the  chemical  composition  ap- 
proaches that  of  normal  cliopside. 

Diopside,  Zillerthal,  Switzerland. .  .2Fna  =  54°  43'  cAc  =  39°       Osann. 

Diopside,  Ducktown,  Term 2Fna  =  54°  32'  cAc  =  40°  19' A.  Schmidt. 

Coccolite,  Arendal,  Norway 2  V     =  58°  38'  C'A  t  =  40°  22' Tschermak. 

Diopside,  Ala,  Tyrol 2  V     =  58°  59'  c  A  t  =  38°  54' . . . .  Des  Cloizeaux. 

Augite,  Bohemia .27     =  59°  28'  cAc  =  46°  40' Osann. 

Diopside,  Nordmarken 2  V     =60°        c  A  t  =  46°  45'  . . .  Tschermak. 

Augite,  Borislau 2V    =61°        cAc  =  45°  30'. . . . 

Hedenbergite,  Tunaberg,  Sweden  ..2 V    =  62°  32'  cAC  =  45°  66'. . . . 
Augite,  Frascati,  Tyrol 27    =68°        CAC  =  54° 

From  Fig.  83  it  is  evident  that  all  sections  of  the  orthodiagonal 
zone  will  show  the  emergence  of  axes  or  bisectrices.  Cleavage  plates 
parallel  to  oP  (001)  or  ooPoo  (100)  exhibit  an  axis  somewhat  eccentric 
to  the  field  of  view,  occasionally  almost  in  its  centre.  The  point  of 
emergence  of  the  acute  bisectrix  is  shown  in  sections  which  correspond 


236  PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

to  a  negative  ortho-hemidome,  that  of  the  obtuse  bisectrix  in  set- 
tions  corresponding  to  a  positive  ortho-hemidorne.  One  or  more 
brightly  colored  axial  rings  are  seen  about  each  axis  even  in  very  thin 
sections  because  of  the  strong  double  refraction  :  this  is  not  the  case  in 
the  orthorhombic  pyroxenes.  The  inclined  extinction  is  clearly  seen 
in  sections  which  are  perpendicular  to  the  acute  bisectrix.  In  the 
diagonal  position  one  hyperbola  has  brilliant  red  on  the  inside  and 
blue  on  the  outside;  on  the  other  hyperbola  the  colors  are  reversed, 
and  are  noticeably  duller 

The  pleochroism  of  monoclinic  pyroxenes  is  usually  small,  especially 
in  thin  sections,  and  in  general  only  shows  itself  as  different  shades  of 
the  body  color,  green  or  brown.  It  may  occasionally,  however,  be  con- 
siderable. Thus  Tschermak  found  i/i  the  black  basaltic  augite  from 
Frascati,  c  olive-green,  b  grass-green,  a  clove-brown.  The  porphyritic 
-augites  of  trachytes,  phonolites,  and  andesites  often  have  b  brownish 
yellow  to  reddish,  ft  and  c  greenish;  they  thus  resemble  the  ortho- 
rhombic 'pyroxenes,  in  which,  however,  d  and  c  show  a  recognizable 
difference  of  color.  In  the  augites  of  tephrite  b  is  often  green  to 
greenish  yellow,  a  and  C  reddish  brown  ;  in  the  titaniferous  augites  of 
basaltic  rocks,  especially  of  the  nepheline  rocks,  b  is  usually  violet,  a 
and  c  yellowish  gray  to  yellowish.  The  differences  of  absorption  in 
the  direction  of  the  principal  vibrations  are  always  small — a  relation 
which  is  to  be  noted  in  contrast  to  that  of  the  hornblendes. 

H.  =  5-6.  The  specific  gravity  of  the  rock-making  pyroxenes, 
when  pure,  is  never  lower  than  3.3.  It  is  lowest  in  the  diopsides  and 
diallages  poor  in  iron,  rises  rapidly  with  the  iron  percentage,  and  reaches 
its  maximum  in  those  pyroxenes  in  which  the  acmite  molecule  abounds, 
when  it  is  3.55.  This  high  specific  gravity  is  important  for  its  mechan- 
ical separation  from  the  amphiboles,  whose  density  is  considerably 
lower  than  that  of  pyroxenes  of  similar  chemical  composition. 

The  chemical  composition  of  the  monoclinic  pyroxenes  is  one 
which  is  not  yet  fully  explained.  According  to  Tschermak's  concep- 
tion, they  consist  of  isornorphous  mixtures  of  the  molecular  combi- 
nations CaMgSi'A,  CaFeSi2O6,  MgAlJSiO.,  MgFeJSiO.,  FeAl2SiO6, 
in  which  a  small  amount  of  manganese  can  replace  iron,  and  with 
which,  moreover,  may  be  combined  the  molecule  NaFeSi2O6,  which 
preponderates  in  acmite.  The  compound  CaMgSiO6  is  present  al- 
most pure  in  the  colorless  diopsides,  CaFeSiaO6  in  hedenbergite ;  the 
sesquioxide-bearing  molecule  is  not  known  by  itself.  The  pyroxenes 
of  the  diopside  and  diallage  series  consist  principally  of  isomorphous 
mixtures  of  the  diopside  .and  hedenbergite  molecules,  with  only  sub- 


MONOCLINIC   PYROXENES.  237 

ordinate  amounts  of  the  sesquioxide-bearing  compound,  whose  abundant 
occurrence  on  the  other  hand  characterizes  the  members  of  the  augite 
series.  There  may  also  be  present  in  variable  amounts,  TiO2,  the  com- 
pound ]$Ta2Al2Si4O8,  besides  Mg2Si2O6  and  Fe2Si2O6. 

The  pyroxenes  generally  fuse  easily  to  glasses  in  which  microscopic 
crystallizations  usually  take  place  if  the  fusion  is  continued.  They  are 
only  attacked  by  hydrochloric  acid  with  difficulty,  or  not  at  all.  The 
results  in  testing  for  the  bases  with  hydrofluosilicic  acid  are  often 
only  reached  upon  repeated  treatment.  The  green  and  yellow  varieties- 
become  red  to  brown  through  the  separation  of  ferric  oxide  upon  being^ 
heated  to  redness  on  platinum  foil. 

The  processes  of  alteration  of  the  monoclinic  pyroxenes  are  very 
different  according  to  their  chemical  composition,  and  to  the  geological 
moments  influencing  them.  Hence  they  will  be  described  under  the 
different  varieties  to  which  they  belong. 

Under  malacolite  will  be  included  those  rock-making  monoclinic 
pyroxenes  which  are  poor  in  alumina  or  free  from  it,  and-  are  not 
laminated  parallel  to  the  orthopinacoid.  This  variety  appears  to  occur 
but  sparingly  in  eruptive  rocks.  It  forms  well-developed  crystals 
in  the  augite  granitites  of  Laveline  in  the  Yosges,  and  also  from 
other  localities.  The  colorless  to  light-green  pyroxenes  of  many 
quartz  porphyries,  and  those  of  kersantite,  probably  belong  to  this 
variety.  Besides  the  perfect  cleavage  parallel  to  the  prism,  they  are 
characterized  by  traces  of  cleavage  parallel  to  the  vertical  pinacoids, 
and  by  a  well-defined  cross  parting,  as  well  as  by  the  easy  alteration  into 
a  greenish  fibrous  aggregate  which  belongs  to  serpentine.  The  altera- 
tion commences  at  the  transverse  cracks,  the  fibres  placing  themselves- 
parallel  to  one  another,  and  perpendicular  to  the  walls  of  the  cracks. 
A  crystal  then  resolves  itself  into  a  row  of  fragments,  each  passing  into 
a  felty  aggregate  of  fibres  with  which  calcite  is  very  often  associated. 

Malacolite  is  very  widely  disseminated  in  the  Archaean  rocks,  being 
chiefly  confined  to  the  granular  limestones,  in  which  it  occurs  partly 
as  isolated  crystals,  partly  in  prismatic  or  granular  aggregates.  From 
the  granular  limestones  it  may  be  traced  to  those  intercalated  rocks 
composed  mainly  of  lime  and  magnesia  silicates  (ophiolites),  found  in 
the  gneisses.  In  such  malacolites  Schumacher  observed  the  paramor- 
phic  alteration  into  amphibole.  Kelated  to  this  is  the  occurrence  of  a 
colorless  monoclinic  pyroxene  in  prismatic  individuals  in  many  amphi- 
bolites  and  gneisses.  The  lime-silicate  hornstones  of  the  granite-schist 
contact  zones  contain  malacolite  quite  abundantly,  together  with  garnet 
and  epidote.  A  confusion  with  the  last-named  mineral  may  be  most 


238          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

easily  avoided  by  observation  in  convergent  light.  In  malacolite  the 
axial  plane  lies  parallel  to  the  longitudinal  axis  and  cleavage  cracks,  in 
epidote  perpendicular  to  the  same  directions.  It  is  distinguished  from 
zoisite  by  its  strong  double  refraction.  The  coarse  malacolite  aggre- 
gates from  Sala,  Sweden ;  Arendal,  Norway ;  Stambach,  Gefrees, 
Bavaria,  etc.,  often  show  an  alteration  into  talc  scales.  Malacolite 
possesses  no  constant  micro-structure ;  the  inclusions  are  chiefly  fluid 
and  gas  interpositions  of  cylindrical  form,  which  are  arranged  parallel 
to  the  cleavage  faces. 

Diallage. — The  chemical  composition  of  the  rock-making  diallages 
is  in  general  the  same  as  that  of  malacolite,  with  a  slight  admixture  of 
the  molecular  group  (Mg,  Fe)O,  (Al,  Fe)aO3,  SiO2,  with  which  is  as- 
sociated the  acmite  molecule,  NaaO,  Fe2O3, 4SiO2,  in  rocks  rich  in  alkali 
{augite  syenites).  Morphologically  they  are  characterized  by  the  al- 
most complete  absence  of  crystallographic  boundary,  and  the  presence* 
of  a  very  distinct  parting  parallel  to  ooJ^oo  (100),  in  addition  to  the 
prismatic  cleavage  (PL  XX.  Fig.  6).  A  much  less  distinct  parting 
parallel  to  oo^Pco  (010)  is  occasionally  observed,  and  very  rarely  one 
parallel  to  oP  (001).  The  modes  of  twinning  are  the  same  as  those  in 
malacolite ;  they  are  mostly  developed  as  polysynthetic  lamellse.  Dial- 
lage is  very  frequently  filled  with  lamellse  of  an  orthorhombic  pyroxene 
(bronzite)  (PL  XVII.  Fig.  6) ;  the  latter  has  the  prism  in  common 
with  diallage,  and  its  ccPoo  (010)  coincides  with  ooPoo  (100)  of  dial- 
lage.  Much  less  frequently  prisms  of  hornblende  are  found  in  diallage 
parallel  to  the  parting  along  ccPab  ,  which  are  probably  primary. 

Very  frequently  the  same  tabular  microscopic  interpositions  occur 
in  diallage  which  have  been  described  at  length  under  bronzite  and 
hypersthene  (p.  206).  They  lie  chiefly  in  the  plane  of  parting,  arranged 
parallel  in  such  a  way  that  they  appear  broad  and  shortened  in  the 
•direction  of  the  prism  axis  in  sections  parallel  to  the  plane  of  parting, 
while  in  sections  parallel  to  ooPoo  (010)  they  appear  narrow  and 
relatively  elongated  in  the  direction  of  the  prism  axis.  They  produce 
the  metallic  sheen  (schiller)  on  transverse  faces.  They  also  occasion- 
ally lie  in  an  inclined  face. 

All  longitudinal  sections  exhibit  more  or  less  distinctly  a  fibrous  to 
prismatic  structure  like  that  of  bronzite;  here  also  it  is  often  united 
with  the  appearance  of  cylindrical  cavities  which  are  frequently  filled 
with  iron  ores,  carbonates,  and  other  decomposition  products.  The 
fibrous  structure  appears  to  be  the  result  of  prismatic  aggregation. 

The  diallages  become  transparent  with  a  grayish-green  to  green 
color,  and  in  many  rocks  brown  ;  index  of  refraction,  double  refraction, 
extinction  angle  (<?  A  C  =  39°  41'),  and  axial  angle  are  nearly  the  same 


MONOCLINIC  PYROXENES.  239 

as  in  the  malacolites.  But  the  axial  angle  often  falls  below  that  which 
is  characteristic  of  the  diopside  series ;  thus  in  diallage  from  Yolpers- 
dorf  2  Va  =  47°  51'.  The  most  important  optical  characteristic  of 
diallage,  especially  in  contrast  to  the  orthorhombic  pyroxenes  and  bas- 
tite,  is  the  eccentric  point  of  emergence  of  an  optic  axis  in  cleavage 
plates  parallel  to  ooP56  (100)  in  convergent  light. 

Pleochroism  is  seldom  observed  in  the  diallages,  and  then  b  is 
usually  yellowish,  a  and  c  greenish.  Differences  of  absorption  are 
scarcely  noticeable  in  thin  section. 

Diallage  is  an  essential  constituent  of  gabbro  and  its  derivatives,  as 
well  as  of  many  peridotites  and  serpentines,  and  forms  in  these  rocks 
lamellar  masses  often  of  considerable  size.  These  enclose  the  older 
constituents  of  these  rocks,  especially  magnetite,  titanic  iron,  chromite, 
and  olivine.  It  is  possible  that  minute  inclusions  of  these  minerals 
furnish  the  small  quantities  of  TiO2  and  Cr2O3  found  in  the  analyses 
of  diallages.  Diallage  is  frequently  surrounded  by  a  parallel  or  irregu- 
lar growth  of  orthorhombic  pyroxene  and  hornblende.  Diallage  occurs 
but  sparingly  in  eruptive  rocks  of  basaltic  or  andesitic  habit.  In  these 
it  forms  prismatic  crystals  in  which  the  parallel  fibrous  structure,  the 
microlitic  interpositions,  and  the  intergrowth  with  bronzite  and  horn- 
blende are  wanting,  but  in  which,  on  the  other  hand,  glass  inclusions  are 
occasionally  present.  In  the  Archaean  rocks  diallage  is  seldom  met 
with.  It  is  here  confined  to  the  olivine  rocks  and  olivine  schists  (so- 
called  chrome  diopside)  and  their  derivatives,  as  well  as  to  certain 
amphibolites,  which  may  be  considered  as  probably  dynamo-metamor- 
phic  gabbros,  and  to  the  rocks  of  doubtful  origin  known  as  trap 
granulites. 

The  alteration  processes  observed  in  diallage  may  be  divided  into 
two  groups.  The  alteration  induced  by  the  taking  up  of  water  pro- 
duces fibrous  and  flaky  aggregates  of  serpentine  or  chlorite,  which  not 
infrequently  preserve  the  microstructure  of  the  parent  mineral,  and 
with  which  are  associated  more  or  less  calcite  and  epidote.  In  contrast 
to  this  process  of  normal  atmospheric  weathering,  the  alteration  to 
amphibole.  minerals  must  be  referred  to  mountain-making  processes, 
since  it  appears  to  be  confined  to  gabbros  in  the  vicinity  of  crystalline 
and  phyllitic  schists.  This  alteration  usually  advances  from  the  pe- 
riphery toward  the  centre,  so  that  in  the  larger  individuals  the  central 
portion  sometimes  remains  unaltered,  and  this  process  is  often  accom- 
panied by  a  considerable  deformation  of  the  laminated  crystalloids  to 
more  or  less  elongated  streaks  (Flaser gabbro).  The  resulting  amphi- 
boles  apparently  belong  to  common  actinolite  and  also  to  smaragdite. 


240          PHYSIOGRAPHY  OF  THE  ROCK-MAKING-  MINERALS. 

Augite. — The  aluminous  monoclinic  pyroxenes  are  among  the  com- 
monest constituents  of  crystalline  rocks.  In  eruptive  rocks  with  por- 
phyritic  structure,  less  frequently  in  those  with  granular  structure,  it 
occurs  in  perfectly  developed  crystals  with  the  form  of  Fig.  79,  and 
then  belongs  to  the  older  secretions  of  the  magma.  This  is  the  case  in 
certain  minettes,  more  rarely  in  quartz  porphyries,  very  frequently  in 
the  qnartzless  porphyries,  porphyrites,  melaphyres,  teschenites,  rhyolites 
(liparites),  trachytes,  phonolites,  andesites,  and  basaltic  rocks.  It  appears 
in  the  form  of  irregular  columns  and  grains  in  the  granular  massive 
rocks,  elseolite  syenites,  augite  diorites,  diabases,  and  picrites,  and  in  the 
porphyritic  rocks  when  it  belongs  to  a  second,  younger  generation  of 
pyroxene  as  a  constituent  of  the  ground  mass.  Zonal  structure  result- 
ing from  isomorphous  lamination  is  uncommonly  wide-spread. 

The  cleavage  parallel  to  the  prism  is  almost  always  very  distinct ; 
besides  this,  pinacoidal  cleavages  occur,  especially  in  the  diabasis,  which 
give  the  augite  a  diallage-like  appearance,  but  never  reach  the  perfec- 
tion of  the  parting  in  the  latter  mineral.  On  the  other  hand,  the 
cleavnge  parallel  to  oo^P  (110)  is  sometimes  so  imperfect  in  many  an- 
gites,  especially  in  basaltic  and  phonolitic  ones,  that  it  is  not  expressed 
microscopically  by  cracks,  and  cannot  be  produced  macroscopically. 

The  colors  by  transmitted  light  are  green  or  brown,  more  rarely 
yellow  to  red  or  violet ;  the  extinction  angle  on  ooPoo  (010)  almost 
always  exceeds  40°.  The  axial  angle  varies  greatly,  and  may  occasion- 
ally fall  below  the  minimum  of  diopside;  in  general,  however,  it  is 
larger  than  that  of  the  diopsides  and  diallages. 

The  chemical  composition  varies  greatly,  especially  in  the  relative 
amounts  of  MgO,  A13OS,  SiO3  and  MgO,  Fe?O3,  SiO2  as  well  as  in  the 
acmite  molecule.  It  appears  as  though  the  latter  entered  largely  into 
the  combination  in  rocks  bearing  nepheline  and  leucite. 

The  augites  of  eruptive  rocks  very  frequently  enclose,  besides  the 
minerals  of  older  origin  associated  with  them  (iron-ores,  apatite,  oli- 
vine  mica),  interpositions  of  glass,  often  in  great  number.  The  latter 
are  mostly  round,  egg-shaped  or  irregularly  formed,  but  also  possess 
the  form  of  their  host.  Fluid  inclusions  are  less  commonly  met  with, 
among  them  liquid  carbon  dioxide.  Gas  interpositions  also  are  not 
uncommon. 

Regular  intergrowths  with  amphibole  minerals  have  been  fre- 
quently observed,  both  having  the  vertical  axis  and  plane  of  symmetry 
in  common  (PL  XXI.  Fig.  1) ;  also  those  with  micas  of  the  biotite 
series,  whose  basal  faces  coincide  with  a  prism  face  of  augite,  espe- 
cially in  diorites,  teschnites,  and  tephrites,  as  well  as  in  elseolite  sye- 


MONOCLINIC  PYROXENES.  241 

nites.  Irregular  intergrowths,  which  may  amount  to  perimorphs,  take 
place  particularly  with  nepheline  in  tephrites  and  ncpheline  rocks, 
when  the  development  of  the  augite  extends  into  the  period  of  the 
nepheline  crystallization. 

The  normal  weathering  of  the  augite  of  eruptive  rocks  generally 
leads  to  the  formation  of  chlorite.  It  usually  commences  from  the 
periphery,  less  frequently  from  spots  within  the  crystals  rich  in  in- 
clusions, and  advances  along  the  cleavage.  Sometimes  there  arise 
parallel  fibrous  and  parallel  flaky  or  felty,  green  aggregates  (PI.  XXI. 
Fig.  2),  with  low  double  refraction,  which  gradually  replace  the  entire 
augite  substance.  This  is  often  dotted  with  strongly  refracting  grains 
and  spines  of  green  epidote,  or  with  small  grains  of  calcite  and  iron- 
ores.  Further  weathering  destroys  the  chlorite,  and  in  its  place 
appears  a  mixture  of  carbonates,  limonite,  clay,  and  quartz.  A  decom- 
position under  the  influence  of  stronger  acids  in  a  fluid  or  gaseous 
condition,  that  is,  a  volcanic  decomposition,  produces  pseudornorphs  of 
opal  or  chalcedony  after  augite  by  the  removal  of  all  the  bases — a 
process  which  appears  to  be  limited  to  the  acid  rocks  of  the  trachyte 
and  andesite  families.  The  alteration  of  augite  into  a  hornblende 
mineral,  urolitization,  is  very  common.  It  occurs  almost  exclusively 
in  augite  diorites  and  diabases,  and  will  be  more  particularly  described 
under  uralite. 

Whether  the  numerous  augite  gneisses  in  many  gneissic  regions 
with  green  monoclinic  pyroxenes  contain  a  true  augite  or  a  deeply 
colored  malacolite,  has  not  yet  been  determined.  The  augitic  constitu- 
ent of  these  gneisses  is  almost  always  green,  very  seldom  brown,  and 
usually  forms  irregular  grains  or  columns  elongated  parallel  to  the 
vertical  axis,  in  which  only  fluid  inclusions  are  observed,  and  these  only 
occasionally.  The  weathering  phenomena  are  most  like  those  of  gran- 
itic malacolite.  The  intergrowth  with  amphibole  minerals  and  the 
alteration  into  the  latter  are  frequently  observed  in  this  variety  also. 

Fassaite. — The  leek-green  and  yellowish-green  varieties  of  common 
augite  called  fassaite  appear  to  be  confined  to  contact  metamorphoses 
of  marly  limestone  near  eruptive  rocks.  ;  ,/; 

Omphacite  is  that  variety  of  light-green  common  augite  occurring 
in  eclogites,  which  is  never  crystallographically  bounded,  and  is  usually 
in  rounded  grains  or  short  columnar  aggregates.  It  shows  itself  to  be 
pyroxene  by  its  cleavage  and  high  extinction  angle.  Here  it  is  fre- 
quently intergrown  with  a  green  hornblende  (smaragdite),  so  that  both 
minerals  have  the  vertical  axis  and  plane  of  symmetry  in  common. 
Many  authors  employ  the  term  omphacite  for  pyroxenes  which  from 
16 


242         PHYSIOGRAPHY  OF  THE  EOCK-MAKING  MINERALS. 

their  cleavage  and  chemical  composition  belong  to  diallage,  and  espe- 
cially for  the  so-called  chrome  diopsides.  Omphacite  often  encloses 
great  quantities  of  rutile  crystals  and  grains,  which  are  so  highly 
characteristic  of  the  eclogites. 

Aemite  and  cegirine  are  monoclinic  pyroxenes  rich  in  soda,  which 
differ  from  the  ordinary  augites  in  many  respects.  The  crystals  are 
almost  always  much  elongated  prisms,  in  which  cvPvo  (100)  and  &>P 
(110),  with  a  cleavage  angle  of  87°,  predominate;  while  oo/^oo  (010) 
is  entirely  wanting  or  is  very  slightly  developed.  Terminal  faces  sel- 
dom occur,  the  individuals  fraying  out,  as  it  were,  at  the  ends.  When 
crystal  boundaries  are  wanting  acmite  and  segirine  form  columns, 
scarcely  ever  grains. 

Twinning  parallel  to  the  orthopinacoid  is  common,  frequently  with 
the  insertion  of  several  lamellae  between  the  larger  halves.  Zonal 
structure,  with  an  alternation  of  brown  and  green  color,  is  not  rare ; 
parallel  growth  with  augite  also  occurs.  Dark  mica  plates  and  am- 
phibole  occur  intergrown  with  them  in  the  same  manner  as  with 
augite. 

The  cleavage  parallel  to  tlie  prism  of  87°  is  always  distinctly  no- 
ticeable ;  pinacoidal  cleavage  parallel  to  oo^Poo  may  reach  great  per- 
fection. 

The  color  by  transmitted  light  is  green,  or  brown  to  brownish  yel- 
low; by  incident  light  the  crystals  of  segirine  are 
always  blackish  green,  those  of  acmite  blackish 
brown.  When  both  colors  occur  in  the  same  indi- 
vicinal  the  peripheral  portions  always  appear  brown, 
the  central  green.  The  index  of  refraction  (fina  in 
segirine  from  Laven  =  1.8084,  Sanger)  and  the 
double  refraction  are  very  strong.  The  character 
of  the  double  refraction  is  probably  negative ;  the 
axial  angle  is  large.  The  plane  of  symmetry  is 
the  axial  plane.  The  orientation  of  the  axes 
of  elasticity  varies  considerably  from  that  in  the  other  monoclinic 
pyroxenes.  The  negative  bisectrix  makes  an  angle  of  4°-5°  with 
the  vertical  axis  in  the  acute  angle  fi  (Fig.  84) ;  in  the  zonally  built 
occurrences,  a  A  6  is  greater  in  the  brown  portions  than  in  the  green. 
The  determination  of  the  extinction  angle  is  facilitated  by  the  twinning 
parallel  to  ccPoo  (100).  The  inclination  of  the  directions  of  extinction 
to  one  another  in  two  lamellae  does  not  exceed  10°.  The  positive 
bisectrix  stands  nearly  perpendicular  to  the  orthopinacoid.  The  ex- 
tinction angle  of  rock-making  aegirine  appears  to  be  somewhat  larger. 


MONOGLINIC  PYROXENES.  243 

The  pleochroism  Is  strong,  suggesting  that  of  hornblende.     There  has 
been  observed  on — 

Acmite,  Porsgrund,  fl  dark  brown  to  green-  b  light  brown  to  t  greenish  yellow 

Norway.  ish  brown.  yellow. 

Acmite,  Ditro tt  dark  brown b   brownish  green,  t   brownish  green 

Hungary.  (F.  Becke) 

^Egirine,  Laven a  pure  green  to  blue-   b  olive-green t  grass-green    to 

green.  yellowish 

^Egirine a  chestnut-brown b   olive-green t  grass-green 

(Tschermak) 

^Egirine,  Sarna fl   blue-green. b  sap-green C  yellowish  green 

Sweden.  (Tornebohm) 

The  absorption  is  distinctly  a  >  ft  >  C,  a  always  being  the  axis  of 
elasticity  lying  nearest  the  prism  axis. 

Sp.  gr.  =  3.5-3.6,  greater  than  for  the  other  monoclinic  pyroxenes. 
Chemical  composition  essentially  Na3O,  Fe2Oa,  4SiO2,  with  variable 
amounts  of  the  diopside,  hedenbergite,  and  augite  molecules.  The 
easy  fusibility  with  a  strong  coloration  of  the  flame  is  very  character- 
istic. 

Acmite  and  segirine  appear  to  be  confined  entirely  to  the  eruptive 
rocks,  and  to  develop  chiefly  in  magmas  rich  in  alkalies.  Thus  they 
occur  in  elseolite  syenite,  phonolites,  leucitophyres,  and  related  rocks ; 
also  in  the  phonolitic  trachytes  of  the  Azores.  The  microstructure  of 
these  acmitic  pyroxenes  is  the  same  as  that  of  the  geologically  equiva- 
lent augites. 

Jadeite,  which  is  of  more  interest  from  an  ethnographic  than  from 
a  petrographic  standpoint,  forms  fibrous  columnar  aggregates,  in  which 
a  prismatic  cleavage  is  noticeable.  The  cleavage  according  to  different 
authors  is  from  85°  20'  to  89°  25',  corresponding  approximately  to  the 
pyroxene  prism.  Arzruni,  however,  calls  attention  to  the  dissimilarity 
of  the  cleavage  faces  and  to  the  unsymmetrical  position  of  the  direction 
of  extinction  with  respect  to  the  cleavage  in  cross-section,  and  places 
jadeite  in  the  triclinic  system,  while  Krenner  refers  it  to  the  mono- 
clinic  system, 

Jadeite  is  colorless,  or  almost  colorless,  with  a  tinge  of  greenish  or 
bluish  green.  The  double  refraction  is  great,  hence  the  brilliant  in- 
terference colors.  The  axial  plane  lies  in  the  plane  of  symmetry^  at 
right  angles  to  which,  according  to  Des  Cloizeaux,  there  is  an  imper- 
fect cleavage ;  the  extinction  angle  is  large,  31°-45°  ;  the  character  of 
the  double  refraction  is  positive,  according  to  Krenner,  who  found 
for  Yellow  2Na  =  82°  48'.  On  the  face  ooP^  is  the  locus  of  an  axis 


244 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


with  finely  colored  rings.  Dispersion  weak,  p  <  v.  The  optical 
orientation  is  analogous  to  that  of  diopside. 

H.  =  7-7.5.  Sp.  gr.  =  3.2-3.4.  Chemical  composition  essentially 
Na2O,  A13O3,  4SiO2;  that  is,  an  acmite  in  which  the  iron  oxide  is  re- 
placed by  alumina.  Fusible  without  difficulty,  coloring  the  flame 
strongly  with  sodium. 

Arzruni  observed  its  paramorphic  alteration  into  amphibole. 


Group  of  Monoclinic  A.mpJiiboles. 
Literature. 

CH.  BABROIS,  Memoire  sur  les  schistes  metamorphiques"de  Tile  de  Groix  (Morbikan). 

Ann.  Soc.  geol.  du  Nord.  Lille.  1883.  XI.  18-71.  cf.  also  Bull.  Soc.  min.  Fr. 

1883.  VI.  289  and  C.  R.  1883.  XCVII.  1446. 

C.  BODEWIG,  Ueber  den  Glaukophan  von  Zermatt.  Pogg.  Ann.  1876.  CXLVIII.  224. 
A.  VON  LASAULX,  Ueber  das  Vorkommen  und  die  mineralogische  Zusammensetzung 

eines  neuen  Glaukophangesteins  von  der  Insel  Groix.  Sitzungsber.  niederrhein. 

Ges.  in  Bonn.  1884.  (3.)  XII. 
A.  MICHEL-LEVY,  De  Temploi  du  microscope  polarisant  &  lumiere  parallele  pour 

1'etude  des  plaques  minces  de  roches  eruptives.    Ann.  Min.  Paris.  1877.  (7.)  XII. 

429-434. 
J.  STRUVEK,  Ueber  Gastaldit,  ein  neues  Mineral.     Atti  R.  Accad.  Lincei.  Roma.  (2.) 

XII. 

G.  TSCHERMAK,  Ueber  Pyroxen  und  Amphibol.     T.  M.  M.  1871.  I.  17. 
—  Mikroskopiscke  Unterscheidung  der  Mineralien  aus  der  Augit-,  Amphibol-  und 

Biotitgruppe.     S.  W.  A.  1.  Abthlg.  1869.  LIX.  Mai. 

Next  to  the  monoclinic  pyroxenes  the  monoclinic  amphiboles  are 
the  most  wide-spread  and  important  of  the  dark-colored  ferruginous 
silicates  occurring  in  rocks.  Their  forms  are  here  referred  to  the  axial 
system,  dil:c=.  0.5318 : 1 : 0.2936,  ft  =  75°  02'.  The  rock-making 


m 


Fig.  85 


m 


\ 


Fig.  86 


amphiboles  exhibit  but  few  forms ;  with  a  constant  prismatic  habit  the 
completely   developed   crystals   are   bounded   in    the   prism  zone  by 


MONOCLINIC  AMPHIBOLES.  245 

m  =  ooP  (110)  with  approximately  124°  30',  J  =  o>P£o  (010),  rarely 
a  =  oo  Pdo  (100);  in  common  hornblende  (Fig.  85)  they  are  terminated 
principally  by  I  =  Poo  (Oil)  with  148°  16',  occasionally  by  p  =  oP 
(001),  in  the  basaltic  hornblendes  (Fig.  86),  by  r  —  P  (111)  with 
148°  30'  and  by  p.  The  terminal  faces  are  wanting  in  the  actinolite 
series  and  generally  in  the  common  hornblendes,  and  the  crystals 
become  jagged  and  irregular  or  frayed  out,  while  the  basaltic 
varieties  usually  appear  in  well-developed  forms.  Hence  cross-sec- 
tions are  acutely  rhombic,  with  a  slight  truncation  of  the  acute 
angles,  seldom  with  both  acute  and  obtuse  angles  truncated.  Lono-i. 
tudinal  sections  parallel  to  ooPoo  (100)  are  lath-shaped,  with  an  obtuse 
pair  of  edges  above  and  below,  or  are  jaggedly  terminated ;  sections 
parallel  to  ooPoo  (010)  are  also  lath-shaped,  with  inclined  terminal  edges, 
or  with  an  obtuse,  unsymmetrical  termination,  or  a  jagged  one.  Through 
the  lack  of  crystallographic  boundaries  in  the  prism  zone  there  arise 
columns,  which  when  much  shortened  become  grains ;  they  are  rare, 
however. 

Twinning  parallel  ooPob  (100)  is  frequent  (Fig.  87); 
the  twinning  plane  is  also  the  composition  plane,  and 
generally  passes  through  the  centre  of  the  crystal,  so  that 
the  twinning  is  not  indicated  by  the  outline  of  the  sec- 
tions. Between  the  two  larger  halves  of  the  twin,  as  in 
the  pyroxenes,  one  or  more  twinned  lamellae  (PL  XXL 
Fig.  3)  are  occasionally  intercalated.  Fis- 8<y 

The  amphiboles  assume  microlitic  dimensions  less  frequently  than 
the  pyroxenes  do ;  they  take  the  form  of  thin  needles,  more  rarely 
that  of  plates,  and  occur  as  inclusions  in  accompanying  minerals,  espe- 
cially in  Archaean  rocks,  or  as  alteration  products  (uralite,  pilite).  In- 
cipient forms  of  growth  are  almost  unknown.  Shelly  or  zonal  struc- 
ture of  isomorphous  layers  is  not  uncommon,  and  follows  the  prism 
{PI.  XXI.  Fig.  4).  Chemical  corrosion  is  confined  to  the  amphiboles 
of  eruptive  rocks;  mechanical  deformations  (bendings,  breakings, 
etc.)  are  found  in  massive  and  schistose  rocks. 

All  amphiboles  are  characterized  by  a  perfect  cleavage  parallel  to  the 
prism,  which  generally  shows  itself  in  thin  sections  by  systems  of  sharp 
cracks  crowded  closely  together.  Hence  in  sections  perpendicular  to 
the  vertical  axis  there  are  two  systems  of  equivalent  cracks  intersecting 
at  124°-125°  (PL  X.  Fig.  5) ;  in  all  sections  of  the  prismatic  zone  the 
cracks  are  parallel ;  in  all  other  sections  the  cleavage  cracks  form 
rhombic  figures,  whose  angles  vary  with  the  position  of  the  section. 
In  some  amphiboles  there  are  indications  of  a  parting  parallel  to  the 


246 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


plane  of  symmetry.  Cross*  observed  a  distinct  parting  parallel  to 
Poo  (101)  in  the  actinolite  of  actinolite  schist  from  Brittany,  and  in 
the  common  green  hornblende  of  a  diorite  from  St.  Brieuc. 

The  rock-making  monoclinic  amphiboles,  with  the  exception  of 
colorless  tremolite  and  blue  glaucophane,  become  transparent  with 
green  or  brown  colors,  the  green  colors  predominating  in  actinolite 
and  almost  exclusively  in  common  hornblende,  while  brown  colors  pre- 
dominate in  the  basaltic  hornblendes.  The  last  named  are  occasionally 
red,  partly,  at  least,  in  consequence  of  secondary  heating.  The  index 
of  refraction,  double  refraction,  and  consequently  the  relief  and  inter- 
ference colors,  are  high,  but  always  appear,  however,  to  be  smaller 
than  for  the  corresponding  varieties  of  pyroxene.  The  optic  axes 
always  lie  in  the  plane  of  symmetry ;  their  angle  varies  considerably. 
The  character  of  the  double  refraction  is  generally  negative,  less  fre- 
quently positive.  The  axis  of  least  elasticity  lies  in  the  acute  axial 
angle  /?,  and  is  in  general  slightly  inclined  to  the  vertical  axis  ;  how- 
ever, the  extinction  angle  varies  from  0°  to  27°,  exceeding  this  value 
in  exceptional  cases.  The  relation  between  the  optical  orientation  and 
the  chemical  composition  has  not  yet  been  made  out.  The  variation 
in  the  optical  characters  is  shown  in  the  following  table : 


Extinction 


Axial  Angle. 


Optical 
Character. 


Index  of 
Refraction. 

Trem olite  from 

Skutterud ftna  =  1.6233 

Tremolite #»  =  1.633 

Actinolite,  St.  Gott- 

hard /3na  =  1.629 

Pargasite ft    =1.64 

Common  Horn- 
blende from  Vol- 

persdorf, ftp  =1.642 

Basaltic  Hornblende 

from  Czernosin. . .  ft     =1.710 
Basaltic  Hornblende 

from  Bilin  (?) 

Basaltic  Hornblende 

from    A  r  a  n  y  e  r 

Berg 

Glaucophane,  Zer- 

matt 

Glaucophane,  He  de 

Groix 

Gastaldite ft     =1.6442 


Michel-Levy  determined  the  difference  y  —  ex.  =  0.0265  for  tremo- 

*Studien  liber  bretonische  Gesteine.  T.  M.  P.  M.  1881.  III.  386-400. 
f  Derived  from  the  angle  measured  in  Canada  balsam,  under  the  assumption  that 
n  =  1.55  for  Canada  balsam. 


16° 
15° 

2F™ 

=  81° 
=  88° 

22' 
16' 

— 

S.  Penfield. 
Des  Cloizeaux. 

15° 
18° 

2Fna 

=  80° 
=  59° 

04' 

+ 

Des  Cloizeaux, 
Tschermak. 

19°  53' 

2F 

=  85° 

+ 

Tschermak. 

1°  40' 

2F 

=  79° 

24' 

- 

Haidinger. 

1°-  2° 

2H 

=  92° 

37' 

- 

Des  Cloizeaux. 

37°  12' 

2Hna 

=  51° 

18' 

+ 

Franzeuau. 

4°  16' 

2zzna 

=  51° 

11' 

- 

Bodewig. 

4° 
6° 

2F« 

=  41° 
=  41° 

22'  f 
26'  f 

__ 

v.  Lasaulx. 
ganger. 

MONOCLIN1C  AMPHIBOLES. 


247 


05- 


lite,  =  0.0240  for  pale-brown  common  hornblende,  =  0.0216  for  glau- 
cophane  from  He  de  Groix,  Brittany.  S.  Penfield  determined  on 
trernoite  from  Skutterud  ana  =  1.6065;  yna  =  1.6340,  hence  y  —  a  = 
0.0275.  The  dispersion  in  the  monoclinic  amphiboles  is  p  <  v. 

The  axis  of  mean  elasticity  coincides  with  the  axis  of  symmetry ; 
the  axis  of  least  elasticity  lies  in  the  acute  axial  angle  /2,  with  variable 
inclination  to  the  prism  axis.  The  greatest  extinction  angle  is  found 
in  common  hornblende,  but  even  here  it  seldom  exceeds  20°.  It  may 
be  assumed  as  the  rule  tl^at  the  angle  c  A  C  is  15°-18°  for  actinolite  and 
common  hornblende, 4°-6°  forglaucophaneand  arfvedsonite,  0°-10°for 
the  basaltic  hornblendes.  Fig.  88  pre- 
sents the  orientation  of  the  axes  of 
elasticity  and  of  the  optic  axes  in  the 
clinopinacoid  for  actinolite  and  normal 
common  hornblende.  All  sections 
from  the  zone  oP:  ooPdb  (001:100) 
between  crossed  nicols  in  parallel  light 
behave  like  sections  from  a  principal 
zone  of  an  orthorhornbic  mineral. 
The  cleavage  cracks  form  rhombs 
whose  obtuse  angle  of  122°  30'  on  oP 
at  first  increases  to  124°  30',  then  de- 
creases to  0°.  The  extinction  lies 
diagonal  to  the  cleavage  cracks,  as  long 
as  these  intersect;  it  is  parallel  and 
normal  to  these  when  they  are  parallel  TCigl  88 

to  each  other. 

In  the  zone  ooPdb  :  ooPoo  (100  :  010)  the  cleavage  cracks  are 
always  parallel  to  one  another;  the  extinction  angle  measured  from 
them  increases  from  0°  on  ooPob  (100)  to  a  maximum  of  the  angle  c/\t 
(0°  to  20°)  on  the  clinopinacoid.  In  the  zone  oP :  ooP^o  (001 : 010) 
the  acute  angle  of  the  cleavage  cracks  decreases  from  57°  30'  on  oP 
(001)  to  0°  on  ooPoo  (010).  The  extinction  lies  diagonal  in  the  section 
parallel  to  0P,  and  unsymmetrical  in  all  other  sections. 

All  sections  in  the  orthodiagonal  zone  show  the  emergence  of 
axes  or  bisectrices  in  convergent  light;  the  interference  figures  in 
general  are  not  so  brilliantly  colored  nor  surrounded  by  so  many  rings 
as  those  of  pyroxene;  and  the  axial  plane  indicated  by  the  straightened 
axial  bar  bisects  the  field  of  view  and  the  cleavage  cracks,  when  these 
intersect  one  another,  and  lies  parallel  to  them  in  sections  parallel 
to  GoPob  (100).  The  dispersion  is  distinctly  noticeable  on  the  hyper- 


248         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

bola  of  the  axis  emerging  from  ooPdb  (100),  but  scarcely  or  not  at  all 
on  that  emerging  from  oP  (001). 

The  behavior  of  sections  parallel  to  the  three  principal  zones  in 
twins  parallel  to  oo^Poo  (100)  differs  according  to  the  value  of  the 
angle  <?AC,  and  may  be  deduced  from  what  has  just  been  said  concern- 
ing the  action  of  a  single  individual  in  these  zones. 

All  amphiboles,  which  are  not  colorless,  possess  a  distinct  pleochro- 
ism,  which  increases  rapidly  with  the  depth  of  the  color,  and  is  spe- 
cially strong  in  the  brown  varieties.  The  colors  are  very  different 
for  the  different  varieties,  and  will  be  described  under  each.  Locally 
stronger  pleochroism  is  sometimes  due  to  a  zonal  change  of  coloring, 
at  other  times  to  pigments,  which  have  concentrated  themselves  about 
inclusions.  Faintly  green  amphiboles  with  little  pleochroism  may 
often  be  permanently  colored  intensely  red,  and  become  strongly  pleo- 
cliroic  by  being  heated  to  redness  on  platinum  foil.  The  differences 
of  absorption  in  the  direction  of  the  three  axes  of  elasticity  are 
generally  very  noticeable :  the  absorption  parallel  to  the  negative 
bisectrix  is  the  weakest;  that  parallel  to  the  positive  bisectrix  the 
strongest ;  the  absorption  parallel  to  the  axis  of  symmetry  is  some- 
times very  great.  Hence  in  general  c  >  b  >  d. 

The  specific  gravity  of  the  amphiboles  is  always  less  than  that  of 
chemically  similar  pyroxenes,  with  the  exception  of  the  varieties  rich 
in  alkali  and  iron  oxide,  which  possess  nearly  the  same  density  in  both 
series.  The  lightest  amphiboles  are  those  free  from  alumina  (sp.  gr. 
=  2.9-3.16)  and  members  of  the  glaucophane  series  (3.05-3.15) ;  the 
hornblendes  proper  have  3.15-3.33,  and  the  density  increases  with  the 
iron  percentage.  This  smaller  density  is  useful  in  the  mechanical  sep- 
aration of  the  amphiboles  and  pyroxenes.  In  general,  the  amphiboles 
are  more  strongly  attracted  by  an  electro-magnet  than  the  pyroxenes 
of  analogous  composition. 

The  chemical  composition  of  the  amphiboles  is  not  as  well  known 
as  that  of  the  pyroxenes.  The  members  of  the  actinolite  series  free 
from  alumina  are  essentially  isomorphotis  mixtures  of  3MgO,  CaO, 
4SiO2,  and  3FeO,  CaO,  4SiO2,  in  which  the  magnesia-lime  molecule 
always  predominates.  In  the  arfvedsonites  occurs  the  molecule  NaO2, 
Fe2O3,  4SiO2,  which  corresponds  to  the  acmite  molecule  of  the  pyrox- 
enes; in  the  glaucophanes  there  is  chiefly  the  jadeite  molecule  E"aQO, 
AlaO3,  4SiO2,  with  variable  amount  of  the  actinolite  molecule.  It  is 
not  yet  certain  how  the  alumina  and  iron-oxide  percentages  of  the  horn- 
blendes proper  are  to  be  expressed  :  they  are  sometimes  considered  as 
analogous  to  the  molecule  (Mg,  Fe)O(Ala,  Fe2)O3SiO2  of  the  pyroxenes, 


MONOCLIJSIC  AMPHIBOLES.  249 

while  others  consider  the  compound  R2O3  as  isomorpkous  with  KSiOs. 
R.  Scharizer  has  undertaken  to  show  that  there  exists  in  the  horn- 
blendes an  isomorphous  mixture  of  the  actinolite  molecule  with  a  mole- 
ule  (R2,  R)3(A1,  Fe)3Si3O12,  which  he  designates  as  the  syntagmatite 
molecule.  In  the  latter,  according  to  his  conception,  the  relation  be- 
tween the  monoxides  is  always  (CaO  +  E2O):  (MgO  +  FeO  +  MnO) 
=  3:4.  The  role  of  the  titanic  acid,  water,  and  fluorine  given  in  many 
analyses  of  amphiboles  is  uncertain.  Unfortunately,  there  is  only  a 
limited  number  of  investigations  of  rock-making  amphiboles. 

Tremolite  occurs  in  columnar  and  lamellar  masses  and  individuals 
in  the  granular  limestone  of  the  Archaean,  in  many  silicate  hornstones, 
and  with  olivine  and  its  alteration  products  in  certain  olivine  rocks  and 
serpentines.  In  the  latter  it  sometimes  occurs  as  an  original  constitu- 
ent, at  other  times  as  a  secondary  one.  The  amphibole  cleavage  and 
strong  double  refraction  in  connection  with  its  colorlessness  fully  char- 
acterize it.  A  transverse  parting  is  quite  common  in  addition  to  the 
cleavage.  The  individuals  often  fray  out  at  the  ends,  and  pass  over 
into  asbestus-like  aggregates.  To  distinguish  it  from  muscovite,  talc, 
and  wollastonite,  with  which  it  may  be  confounded  in  certain  sections,  it 
should  be  investigated  in  convergent  light.  In  tremolite  the  axial  plane 
lies  parallel  to  the  cleavage,  in  the  others  normal  to  it.  Tremolite 
appears  as  an  alteration  product  in  the  form  of  a  marginal  border  about 
olivine  in  many  Scandinavian  olivine  diabases  and  olivine  gabbros. 
Tremolite  usually  alters  into  talc,  the  talc  scales  penetrating  the  tremo- 
lite substance  by  degrees  from  the  periphery,  from  fissures  and  cleavage 
cracks,  until  in  certain  stages  of  the  process  it  lies  in  the  form  of  oblong 
and  acutely  rhombic  meshes  within  a  net  of  talc  scales. 

Actinolite  also  forms  prismatic  individuals  or  columnar  and  fibrous 
aggregates,  on  which  terminal  faces  never  occur.  It  is  distinguished 
from  tremolite  by  its  more  or  less  green  color.  Besides  the  prismatic 
cleavage,  there  is  occasionally  one  parallel  to  coPco  (010).  The  separa- 
tion of  the  columns  at  right  angles  to  their  axis  is  common.  When  but 
slightly  colored  and  in  thin  sections  the  pleochroism  is  scarcely  notice- 
able ;  when  more  strongly  colored,  the  absorption  is  distinct,  c>  b  >  a, 
even  when  all  the  rays  are  green  ;  or  there  may  be  a  yellowish  tone  in 
rays  vibrating  parallel  to  fc  and  a,  while  the  color  parallel  to  c  is  green. 
Actinolite,  like  tremolite,  is  free  from  inclusions  of  the  minerals  asso- 
ciated with  it.  The  real  home  of  actinolite  is  in  the  Archaean,  where 
it  forms,  either  alone  or  in  combination  with  pyroxene,  epidote,  or 
chlorite,  the  varied  series  of  actinolite  schists ;  it  occurs  with  quartz 


250         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

and  albite  in  many  green  schists,  and  as  an  accessory  in  chlorite  and 
talcose  schists.  It  appears  as  a  secondary  constituent  in  diabases  and 
schalsteins  and  in  gabbros,  and  in  these  is  an  alteration  product  of  pyrox- 
ene or  occasionally  of  olivine.  The  emerald-green  actinolite,  which  oc- 
curs in  the  so-called  saussurite  gabbros,  is  called  smaragdite.  It  forms 
very  delicate  columnar  aggregates  of  a  pale  greenish  white  color  by 
transmitted  light,  which  are  only  transparent  when  very  thin — a  con- 
sequence of  the  delicate  aggregation.  Smaragdite  aggregates  often 
appear  in  the  form  of  diallage,  as  pseudomorphs  or  probably  as  para- 
morphs  after  the  diallage.  Actinolite  does  not  occur  as  a  primary  con- 
stituent of  eruptive  rocks.  Decomposition  processes  are  seldom  ob- 
served in  actinolite ;  it  passes  into  fibrous  and  scaly  cryptocrystalline 
aggregates  with  a  green  color  which  may  belong  to  serpentine.  The 
calcium  component  is  usually  secreted  as  calcite  in  small  grains  and 
rounded  masses. 

Nephrite  or  jade  is  a  felty,  fibrous  actinolite  with  a  more  or  less 
obscure  schistose  structure.  There  are  undoubted  occurrences  of  it  on 
Batugol  Mountain  in  Eastern  Siberia,  in  theKuenluen,  in  New  Zealand. 
Traube  appears  to  have  found  an  occurrence  at  Jordansnriihl  intimately 
associated  with  serpentine  and  granulite. 

Common  hornblende  only  forms  regularly  bounded  crystals  in  those 
old  eruptive  rocks  with  porphyritic  structure,  for  example,  in  certain 
granite  porphyries,  syenite  porphyries,  and  diorite  porphyrites ;  in  the 
granular  massive  rocks  of  the  older  formations  and  in  the  Archaean  it 
appears  in  more  or  less  distinctly  prismatic  individuals,  less  frequently 
in  plates  or  grains.  A  peculiar  variety  is  the  so-called  reedy  ("  schilfige") 
hornblende,  which  consists  of  approximately  parallel  columnar  to  fibrous 
amphibole  aggregates  with  a  light-green  color  and  slight  pleochroism, 
and  which  is  usually  mixed  with  epidote,  and  chlorite,  and  is  common 
in  certain  eruptive  rocks  of  the  diabase  series  and  in  many  amphibolites. 
In  many  cases  it  can  be  shown  to  have  originated  from  augite,  and  this 
is  probably  true  for  all  its  occurrences.  It  is  therefore  a  uralitic  horn- 
blende, and  its  characters  are  more  closely  related  to  actinolite  than  to 
common  hornblende.  In  distinction  to  these  reedy  aggregates  the  horn- 
blendes proper  are  termed  compact. 

Common  hornblende  is  mostly  colored  green  ;  it  is  deep  brown  to 
brownish  red  in  tonalite,  in  many  diorite  porphyrites,  in  teschenites ; 
less  frequently  in  the  diorites,  gabbros,  and  Archaean  rocks.  Green 
hornblende  has  an  extinction  angle  like  that  of  actinolite,  or  still  higher ; 
in  cleavage  plates  parallel  to  the  prism  t/\c  =  13°  or  more.  The  pleo- 
chorism  is  confined  to  green  tones,  and  only  those  rays  vibrating  parallel 


MOXOCLINIC  AMPIIIBOLES.  251 

to  a  occasionally  appear  yellow.  The  green  parallel  to  b  often  Las  a 
tinge  of  brown,  that  parallel  to  c  a  tinge  of  blue.  Brown  hornblende  is 
generally  more  pleochroic  than  the  green  :  the  colors  along  c  and  b  are 
brown  in  different  shades ;  a  is  yellowish  or  rarely  greenish,  The  angle 
C/\c  is  smaller;  on  cleavage  plates  parallel  to  oojP  (110)  the  extinction 
angle  is  at  most  13°,  and  may  fall  almost  to  0°. 

Common  hornblende  possesses  no  constant  microstructure ;  it  gen- 
erally  encloses  the  ores  and  apatite,  or  other  minerals  associated  with  it 
which  are  older  than  it  is.  In  many  eruptive  rocks  it  carries  the  inter- 
positions characteristic  of  hypersthene  and  diallage,  in  the  Archaean  rocks- 
it  often  contains  rutile.  Parallel  intergrowths  with  pyroxene  are  fre- 
quent ;  the  hornblende  usually  lies  peripherally  about  the  pyroxene, 
having  the  axes  b  and  c  in  common.  More  rarely  the  pyroxene  sur- 
rounds the  hornblende  (in  some  elseolite  syenites)  and  then  appears  to- 
have  been  derived  from  the  hornblende  by  rnagmatic  processes.  Thus 
in  the  granular  eruptive  rocks  the  formation  of  pyroxene  appears  ta 
have  preceded  that  of  amphibole.  When  it  is  intergrown  with  biotite, 
the  latter  appears  to  have  been  the  older,  and  generally  lies  with  its 
base  on  the  cleavage  faces  of  the  hornblende. 

The  alteration  of  hornblende  to  chlorite,  with  the  secretion  of  epi- 
dote  or  calciteand  quartz,  is  a  wide-spread  process  of  weathering  ;  since 
the  chlorite  may  further  alter  into  a  mixture  of  carbonates,  clay, 
limonite,  and  quartz,  there  arise  pseudomorphs  of  these  minerals  after 
amphibole.  The  hornblende  frays  out  or  becomes  fibrous  during  the 
chloritization,  and  since  the  chlorite  scales  accumulate  from  the  periph- 
ery and  cleavage  faces,  such  a  pseudomorph  may  closely  resemble  an 
aggregate  of  reedy  hornblende.  They  may  be  distinguished  by  treat- 
ment with  acid,  with  which  chlorite  gelatinizes,  while  hornblende  is 
not  attacked,  or  at  most  gives  up  iron  to  the  reagents. 

Basaltic  hornblende  is  almost  always  well  crystallized ;  when  the 
outward  form  is  wanting  it  is  evident  that  it  has  been  lost  through 
mechanical  processes  or  magmatic  resorption.  The  cleavage  shows  a 
high  degree  of  perfection,  and  the  cleavage  faces  have  a  high  lustre.  It 
is  black  by  incident  light,  brown  by  transmitted  light  in  most  every 
instance,  usually  in  deep  tones.  A  green  color  occasionally  arises  from 
chemical  alteration,  and  produces  a  decided  diminution  in  the  lustre. 
An  isomorphous  lamination  is  not  uncommon,  in  which  differently 
colored  zones  alternate  with  one  another,  usually  in  shades  of  brown, 
rarely  brown  and  green  ;  they  are  always  few  in  number,  mostly  con- 
sisting of  a  kernel  and  shell.  The  pleochroism  is  almost  always  very 
Strong,  and  varies  from  dark  brown  for  c  to  light  yellow  for  a ;  occa- 


252         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

sionally  a  is  greenish.  The  absorption,  c  >  ft  >  a,  common  to  all  am. 
phiboles,  reaches  its  maximum,  and  at  times  is  as  intense  as  that  of 
biotite.  The  extinction  angles,  with  few  exceptions  (Arany),  are 
.small,  and  may  fall  as  low  as  0°. 

Basaltic  hornblende  is  confined  to  porphvritic  eruptive  rocks,  and 
forms  crystals  in  them,  which  are  among  the  oldest  secretions  of  the 
magma.  Hence  glass  inclusions  are  frequent ;  besides  inclusions  of 
the  ores,  apatite,  biotite,  olivine,  and  other  older  constituents.  Inter- 
growths  with  pyroxene  occur,  the  latter  lying  peripherally,  and  being 
younger  than  the  hornblende.  The  alterations  produced  by  the  action 
of  the  atmosphere  and  of  thermal  waters  are  the  same  as  those  of 
common  hornblende,  and  lead  to  the  formation  of  chlorite,  car- 
bonates, limonite,  and  quartz.  Quite  different,  however,  are  certain 
alterations  which  can  only  be  explained  as  the  result  of  resorbing 
actions  of  the  magma.  The  outlines  of  hornblende  crystals  in  porphy- 
rites,  trachytes,  phonolites,  and  andesites,  as  well  as  in  basalts  and  teph- 
rites,  are  variously  rounded  and  melted  down ;  and  immediately  sur- 
rounding the  crystal  lies  a  dark  zone,  which  in  most  cases  is  formed  of 
opaque  grains  of  ore  and  columns  or  grains  of  augite :  the  latter  not 
infrequently  lie  parallel  to  one  another  and  to  the  hornblende  crystal. 
That  this  aggregation  of  augite  and  opaque  grains  is  the  result  of  a 
magmatic  paramorphism  of  hornblende,  is  shown  by  the  fact  that  it  may 
completely  replace  the  hornblende  crystal  without  changing  its  form. 
This  alteration  belongs  to  a  period  in  the  development  of  the  rock 
when  hornblende  was  no  longer  capable  of  existing  in  the  magma,  and 
became  melted  and  transformed  into  augite,  probably  accompanied  by 
the  separating  out  of  an  iron  oxide.  Very  rarely  the  resorption  of 
basaltic  hornblende  appears  to  be  followed  by  a  new  formation  of  the 
;same,  which  then  surrounds  the  older  secretion  in  the  form  of  microlites. 

Arfvedsonite  forms  columnar  individuals  in  many  elseolite  syenites, 
and  in  the  south  Norwegian  augite  syenites;  it  forms  perfectly  developed 
•crystals  in  certain  phonolites  and  leucitophyres.  Its  colors  are  brown 
and  green.  The  pleochroism  and  absorption  are  strong,  and  vary,  as  in 
basaltic  hornblende,  between  deep  dark  brown  and  yellow  for  the  brown 
varieties,  and  between  deep  olive-green  to  blue-green  and  muddy  yel- 
lowish green  for  green  varieties.  The  extinction  angle  is  rather 
higher  than  for  basaltic  hornblendes  of  the  same  intensity  of  color.  It 
is  further  distinguished  from  the  latter  by  its  higher  specific  gravity 
.and  strong  sodium  reaction,  together  with  its  very  easy  fusibility. 

Glaueophane  always  forms  prismatic  individuals  which  are  bounded 
by  oo  P  (110),  occasionally  by  ooPao  (010)  or  ooPi  (100),  and  pos- 


MONOCLINIC  AMP11IBOLES.  253 

sess  no  terminal  faces.  The  perfect  cleavage  parallel  to  the  prism  with 
the  amphibole  angle  (124°  25'-124:0  44')  and  the  blue  color  by  incident 
light  make  it  easily  recognizable.  It  is  also  characterized  by  a  trans- 
verse parting.  Its  place  in  the  amphibole  series  corresponds  nearly  to- 
that  of  jadeite  in  the  pyroxene  series.  The  extinction  angle  is  very 
small,  4°-6°,  in  the  plane  of  symmetry.  The  pleochroism  is  very  strong 
and  fine:  C  =  sky-blue  to  ultramarine-blue,  seldom  blue-green;  b  = 
reddish  violet  to  bluish  violet ;  a  =  almost  colorless  to  yellowish  gray. 
Sp.  gr.  =  3.0-3.1. 

Glaucophane,  with  which  should  be  classed  gastaldite,  is  almost  ex- 
clusively confined  to  the  Archaean  rocks,  occurring  in  mica  schists,  eclo- 
gite,  and  phyllitic  gneiss.  An  asbestus-like  glaucophane  (crocidolite) 
has  been  found  in  contact-metamorphosed  limestones  of  Breuschthal  in 
the  Yosges.  The  paragenesis  of  glaucophane  is  the  same  as  that  of 
actinolite  and  common  hornblende ;  it  is  associated  with  diallage,  om- 
phacite,  garnet,  epidote,  mica,  and  rutile.  Its  occurrence  in  a  rairiette 
in  the  neighborhood  of  "Wachenbach  in  Breuschthal,  Yosges,  is  excep- 
tional. 

Appendix. —  Uralite  is  a  paramorph  of  amphibole  after  pyroxene,, 
having  the  crystal  form  of  the  latter,  and  the  physical  characters  and 
usually  the  cleavage  of  the  former.  It  appears,  though,  that  in  this- 
transformation  a  part  of  the  lime  separates  out,  for  finely  divided  calcite 
or  epidote  often  accompanies  these  paramorphs. 

The  alteration  of  augite  into  hornblende  usually  proceeds  from  the 
periphery  toward  the  centre  and  from  the  cracks  inward,  so  that  within 
the  uralite  there  are  often  remnants  of  unaltered  augite.  In  this  process 
the  vertical  axis  and  the  axis  of  symmetry  of  the  parent  mineral  remain 
the  same  for  the  new  one.  The  uralite,  however,  does  not  form  a  single 
compact  crystal,  but  consists  of  numerous  slender  columns  exactly  par- 
allel to  one  another.  Cross-sections  exhibit  the  hornblende  cleavage 
traversing  the  whole  extent  of  the  section  (PI.  XXI.  Fig.  5),  while 
longitudinal  sections  appear  finely  fibrous  (PL  XXI.  Fig.  6).  If  the 
original  augite  individual  was  twinned  parallel  to  oojPoo  (100),  then 
columns  of  uralite  along  the  original  composition  plane  stand  in  twinned 
position  to  one  another. 

Uralite  is  always  green,  and  exhibits  the  pleochroism  of  common 
green  hornblende,  C  and  b  green,  ft  yellowish  green.  The  specific 
gravity  is  that  of  hornblende. 

Uralite  is  common  in  diabases,  diabase  porphyrites,  and  related  rocks 
when  these  lie  imbedded  in  faulted  schists.  It  is  also  found  in  many 
augite  diorites  and  augite  syenites.  It  is  in  general  absent  from  the 


254         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

younger  augite  rocks,  but  occurs  in  these  whenever  they  have  been 
subjected  to  the  same  mechanical  processes  which  the  palaeozoic  masses 
have  undergone.  Whether  the  uralite  belongs  to  common  hornblende 
or  to  actinolite  depends  on  the  original  composition  of  the  parent 
pyroxene  mineral. 

The  Mica  Group. 

Literature. 

M.  BAUER,  Untersuchungen  iiber  den  Glimmer  und  verwandte  Mineralien.     Pogg. 
Ann.  1869.  CXXXVIII.  337-370. 

—  Ueber  einige  physikalische  Yerhaltnisse  des  Glimmers.     Z.    D.    G.    G.    1874. 

XXVIII.  137-186. 
E.  REUSCH,  Ueber  die  Kornerprobe  am  zweiaxigen  Glimmer.     Pogg.  Ann.  1869. 

CXXXVI.  130  and  632. 
G.  TSCHEKMAK,  Mikroskopische  Unterscheidung  der  Mineralien  aus  der  Augit-, 

Amphibol-  und  Biotitgruppe.     S.  W.  A.  1869.  LIX.  May  number. 

—  Die  Glimmergruppe.     S.  W.  A.  1877.  LXXVI.  and  1878.  LXXVIII ;  also  Z.  X. 

1878.  II.  14-49  and  1879.  III.  122-167. 

The  micas  are  distinguished  from  all  other  monoclinic  minerals  by 
the  fact  that  in  the  form  of  their  crystals,  and  in  their  optical  behavior, 
they  approach  very  closely  to  hexagonal  or  orthorhombic  substances ; 
in  many  instances  it  is  still  practically  impossible  to  prove  their  mono- 
<ifinic  nature. 

Rock-rnaking  micas  when  they  exhibit  an  outward  crystal  form 
appear  almost  exclusively  in  thin  hexagonal  plates  whose  plane  angles 
are  exactly  120°.  Less  frequently  these  plates  reach  a  thickness  of 
several  millimetres ;  in  this  case  it  can  be  shown  that  only  one  of  the 
three  pairs  of  vertical  faces,  namely,  I  —  ooPcx>  (010),  stands  at  right 
angles  to  the  basal  plane.  It  is  seldom  possible  to  determine  the  inclina- 
tion of  the  other  pairs  of  faces  accurately.enough  to  indicate  them  crys- 
tallographically.  The  faces  most  frequently  met  with  on  micas  of  the 
biotite  series  are:  c  =  oP (001),  b  —  oo  P ^  (010),  m  =  P(lll),  o  = 
-  \P  (112)  (Fig.  89).  An  orthodome  mPoo  (hoi)  and  faces  in  the  zone 
oP :  ooP3  are  very  rare.  Occasionally,  however,  the  orthodome  and 
clinopinacoid  predominate  to  such  an  extent  that  in  many  rhyolites, 
trachytes,  and  andesites  the  mica  plates  appear  to  be  rectangular,  as 
O.  Miigge*  has  observed  in  the  hornblende  andesites  of  the  Azores. 
On  micas  of  the  phlogopite  and  muscovite  series  the  faces  M=  2P 
(221)  and  a  clinodome  are  more  commonly  observed.  The  most  im- 

*  Petrographische  Untersuchung  an  den  Gesteinen  der  Azoren.  K  J.  B.  1883. 
II.  222. 


THE  MICAS. 


255 


portant  angles  are  c  /\o  =  73°  02',  c  A  m  =  81°  19',  c/^M=  85°  38', 
c  frb  =  90°,  m  frni  =  59°  16' for  the  meroxenes,  and  but  slightly  dif- 
ferent for  the  muscovites. 

Mica  plates  from  porphyritic  rocks  often  exhibit  re-entrant  angles 
on  the  faces  in  the  nearly  vertical  zone,  which  result  from  a  twinning 
in  which  the  individuals  are  symmetrical  to  a  left  or  right  prism  face. 
The  commonest  mode  of  composition  is  that  in  which  the  twinned 
individuals  join  along  their  basal  planes  (Fig.  90).  It  also  frequently 
happens  that  two  or  more  individuals  penetrate  one  another  quite 
irregularly,  so  that  a  thin  cleavage  plate  consists  of  two  or  three  indi- 
viduals whose  boundaries  toward  one  another  are  irregular  lines.  In 
many  rocks,  especially  the  minettes,  the  mica  plates  are  elongated  in 


ig.  8Q 


ITig.  9O 


i  91 


the  direction  of  a  diagonal,  and  when  twinned,  the  separate  individuals 
project  laterally,  as  indicated  in  Fig.  91.  The  composition  plane  of  the 
twins  is  very  rarely  a  lateral  face.  Twins  with  common  terminal  faces, 
in  which  the  individuals  are  turned  30°  to  one  another,  are  quite  rare, 
the  twinning  plane  being  in  the  zone  oP\  ccP3  (001 : 130).  The  cross- 
sections  of  mica  crystals  and  twins  parallel  to  the  base,  therefore,  are 
hexagonal,  very  rarely  lath-shaped ;  when  perpendicular  or  inclined  to 
it  they  are  more  or  less  narrow  lath-shaped. 

The  dimensions  of  mica  crystals  sink  to  microscopic  proportions ; 
incipient  forms  of  growth  and  skeleton  crystals  do  not  occur.  How- 
ever, parallel  growths  of  very  small  plates  forming  larger  crystals  are 
met  with,  especially  in  glassy  rocks. 

In  many  rocks  the  micas  possess  no  crystallographic  boundaries 
except  the  basal  planes ;  they  then  form  variously  notched  and  jagged 
plates,  or  parallel  and  rosette-like  aggregates  which  may  grow  to  shells 
and  balls.  Sections  parallel  to  the  face  of  such  plates  then  are  irregular 
lateral  ones  always  lath-shaped.  Zonal  structure  or  isomorphous  lamina- 
tion is  not  uncommon  in  the  dark  micas  of  the  biotite  and  phlogopite 
series ;  from  this  it  is  evident  that  the  growth  follows  the  lateral  faces, 


256 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


sometimes  the  base.  In  the  first  instance  the  bands  of  growth  form 
concentric  hexagons;  in  the  second,  parallel  lines.  The  intergrowth  of 
different  varieties  of  mica  (biotite  and  mnscovite)  with  one  another 
follows  the  same  directions.  Sometimes  muscovite  surrounds  biotite 
like  a  mantle,  at  other  times  it  lies  on  its  upper  and  lower  sides;  the 
biotite  is  always  inside  and  the  muscovite  out. 

Chemical  corrosion  occurs,  especially  on  the  older  secretions  of 
biotite  in  porphyritic  rocks,  and  is  usually  in  the  form  of  a  marginal 
alteration,  which  will  be  described  in  another  place.  Mechanical 
deformations  are  common  to  all  varieties  of  mica,  and  consist  of 
bending,  slipping  along  the  gliding  plane,  curving  of  the  crystals  and 
the  rolling  out  of  the  same.  The  first  two  kinds  of  deformation 
are  particularly  common  in  the  secretions  of  porphyritic  eruptive 
rocks  (PI.  IY.  Fig.  5).  Micas  whose  plates  have  been  completely 
rolled  out  until  they  form  a  row  of  elongated  scales  are  chiefly  met  with 
in  granitic  rocks  of  highly  faulted  mountains  and  in  the  Archaean  rocks. 
All  micas  cleave  very  perfectly  along  the  basal  plane,  and  the 
cleavage  plates,  when  sufficiently  thin,  are  elastic.  Hence  basal  sec- 
tions show  no  cleavage  cracks,  but  all  others  exhibit  very  sharp  and 
abundant  cleavage  lines,  which  are  parallel  to  themselves  and  to 
the  sides  of  the  lath-shaped  sections  (PI.  X.  Fig.  6).  The  elasticity  of 
the  plates  is  greatest  in  the  muscovites,  decreases  almost  to  brittleness 

in  the  phlogopites  and  bio- 
tites,  and  disappears  rapidly 
upon  the  alteration  of  the 
b  last-named  mica  into  chlorite 
aggregates,  giving  place  to 
ordinary  flexibility.  This 
perfect  basal  cleavage  is  one 
of  the  most  important  diag- 
appearing  to  the  same  ex- 
and  chlorites.  There  are 


Fig.  93 a, 


Fig.  Q&  Tt> 

nostic  characters  of  the  mica  minerals, 
tent  and  kind  only  in  the  chloritoids 
other  cohesion  minima  in  mica  which  are  of  diagnostic  importance. 
They  may  be  detected  by  striking  the  mica  plate  a  quick,  elastic  blow 
with  a  needle  point,  when  there  will  appear  about  the  point  struck  a 
six-rayed  star,  the  rays  or  cracks  intersecting  at  60°  (Fig.  92)  and  lying 
parallel  to  the  edge  c :  b  and  to  the  edges  c :  m.  The  first  ray  which  lies 
parallel  to  the  projection  of  the  plane  of  symmetry  on  the  basal  plane 
is  called  the  characteristic  or  leading  ray.  This  figure  is  of  great 
importance  in  the  optical  determination  of  the  micas,  and  is  known  as 
the  percussion  figure. 


THE  MICAS.  257 

If,  on  the  other  hand,  the  mica  p]ate  be  pressed  by  a  dull-pointed 
instrument  without  being  pierced,  there  arises  another  six-rayed  star, 
o?  pressure  figure,  whose  rays  intersect  at  60°.  The  rays  of  the  pres- 
sure figure  are  so  placed  that  they  stand  at  right  angles  to  those  of  the 
percussion  -figure,  each  to  each.  In  Fig.  92  the  pressure  figure  is 
dotted,  its  rays  lie  parallel  to  the  edges  oP :  mP^>  and  oP :  ooP3 ; 
they  are  not  sharp,  but  fibrous,  and  generally  spread  out  in  tufts.  The 
pressure  figure  is  often  incomplete,  one  or  even  two  of  the  rays  failing 
to  appear.  Lines  parallel  to  the  pressure  figure — that  is,  normal  to  the 
ordinary  boundary  of  the  mica  plates — faults,  and  planes  of  separation 
parallel  to  these  lines,  are  very  common  in  rock-making  micas  (PI.  IV. 
Fig.  5),  and  are  apparently  due  to  mountain  pressures.  Moreover, 
regularly  interposed  crystals  of  foreign  bodies  are  usually  arranged 
parallel  to  the  rays  of  the  pressure  figure. 

The  micas  become  transparent  in  very  different  colors  according  to 
their  chemical  composition :  the  members  of  the  muscovite  and 
phlogopite  series  are  colorless  to  light  yellowish  or  light  greenish, 
and  often  exhibit  in  basal  sections  beautifully  iridescent  fiakes  and 
circles  produced  by  numerous  minute  scales  loosened  in  the  grinding. 
The  rock-making  biotites  are  deep  brown  or  green,  also  red  to  almost 
opaque.  The  index  of  refraction  is  not  large ;  the  double  refraction, 
however,  is  very  strong :  both  appear  to  increase  with  the  iron  per- 
centage. Bauer  determined  on  muscovite  a  =  1.537,  ft  =  1.541, 
y  =  1.5T2;  therefore,  y  —  a  =  0.035.  Michel-Levy  found  on  the  mus- 
covite of  granite  from  Montchanin  (Saone-et-Loire),  y  —  a  =  0.035  ;  on 
meroxene  from  Somma,  y  —  a  =  0.0404;  on  biotite  from  Pranal, 
Auvergne,  y  —  a  =  0.060.  Haidinger  determined  on  a  Brazilian  mica 
(apparently  muscovite)  for  the  ray  vibrating  at  right  angles  to  the 
cleavage,  n  =  1.581 ;  for  that  parallel  to  it,  n  =  1.613.  The  necessarily 
brilliant  interference  colors  are  important  in  distinguishing  the  micas 
from  the  faintly  doubly  refracting  chlorites. 

All  micas  are  optically  negative,  and  the  acute  bisectrix  a  is  always 
about  normal  to  the  cleavage  face  oP\  generally,  its  divergence  from 
the  normal  to  oP  is  scarcely  measurable,  but  in  many  micas  reaches 
9°.  In  some  micas  (meroxene,  lepidomelane,  phlogopite,  zinnwaldite) 
the  axial  plane  coincides  with  the  plane  of  symmetry;  its  trace  on  the 
basal  plane  is  parallel  to  the  leading  ray  of  the  percussion  figure 
and  is  normal  to  a  ray  of  the  pressure  figure;  these  kinds  of  mica 
are  called  "  mica  of  the  second  order "  (Fig.  92&).  In  muscovite, 
lepidolite,  phengite,  paragonite,  and  anomite  the  axial  plane  is  normal 
to  the  plane  of  symmetry,  and  its  trace  on  oP  is  normal  to  the  leading 


258          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

ray  of  the  percussion  figure ;  they  are  called  "  mica  of  the  first  order" 
(Fig.  920).  The  axial  angle  varies  from  almost  0°  in  lepidomelane, 
many  biotites  and  anomites,  to  80°  in  many  muscovites. 

Since  the  first  bisectrix  a  is  nearly  normal  to  the  basal  plane, 
the  second  bisectrix  c  in  mica  of  the  first  order  nearly  coincides 
with  the  clinodiagonal  #,  the  axis  of  mean  elasticity  exactly  with  the 
axis  of  symmetry  ;  in  mica  of  the  second  order  fc  nearly  coincides  with 
a,  c  exactly  with  l>.  From  this  it  follows  that  in  all  but  basal  sections 
the  extinction  between  crossed  nicols  is  parallel  and  normal  to  the 
cleavage.  Indeed,  an  inclined  extinction  is  but  seldom  observed,  and 
is  most  noticeable  in  sections  of  twins  more  or  less  inclined  to  the 
cleavage  face.  Basal  sections  of  mica  are  more  distinctly  doubly 
refracting  the  larger  the  axial  angle,  and  approach  more  closely  to  the 
isotropic  behavior  in  parallel  light  the  nearer  the  angle  2  V  is  to  0°. 
Hence  basal  sections  of  biotite  are  often  not  noticeably  doubly  refract- 
ing, but  apparently  isotropic,  and  the  characteristic  feature  in  the 
optical  behavior  of  the  micas  is  that  the  biotites  closely  approach 
the  hexagonal  system,  the  muscovites  and  phlogopites  the  ortho- 
rhombic. 

The  phenomena  in  convergent  light  are  quite  analogous  to  the 
foregoing:  every  cleavage  plate  furnishes  an  axial  figure — in  the  dark- 
colored  biotites  a  dark  cross,  which  scarcely  opens  during  rotation,  often 
not  noticeably,  and  whose  locus  is  usually  in  the  centre  of  the  field  of 
view.  In  the  light-colored  micas  the  interference  figure  is  apparently 
that  of  an  orthorhornbic  mineral  cut  at  right  angles  to  the  negative 
bisectrix.  The  size  of  the  axial  angle  and  the  dispersion  will  be  given 
under  the  different  varieties. 

The  colored  micas  are  strongly  pleochroic,  arid  in  all  of  them 
the  rays  vibrating  parallel  to  the  cleavage  are  far  more  strongly 
absorbed  than  those  normal  to  it.  The  absorption  and  colors  are 
different  for  each  variety  of  mica,  and  will  be  described  under  each 
variety.  Pleochroic  halos  sometimes  occur  around  microscopic  inter, 
positions  in  all  kinds  of  micas ;  they  always  exhibit  the  minimum  of 
darkness  when  the  light  vibrates  perpendicularly  to  the  cleavage. 

The  specific  gravity  varies  with  the  composition,  between  2.75-3.2, 
and  is  difficult  to  determine  with  accuracy  on  account  of  the  tabular 
form  of  the  mica  and  of  the  difficulty  in  moistening  the  plates  with 
fluids.  Consequently,  flakes  of  mica  in  a  rock  powder  remain  suspended 
in  a  heavy  solution  of  much  lower  density  than  that  of  the  mica  itself. 

The  chemical  composition  of  rock-making  micas  has  been  only 
slightly  investigated.  Following  Tschermak's  theory  of  the  constitu- 


THE  MICAS.  259 

tion  of  micas,  they  consist  of  the  isomorphous  molecules  Si6Al6K6O24 
=  K  and  Si6Mg12O24=:  M,  either  alone  or  in  combination  ;  with 
which  in  some  varieties  is  associated  the  compound  SiJOHBO,4=  S 
or  Si10O8Flw=  S'.  Titanium  may  enter  into  the  combination  to 
a  considerable  extent;  alumina  is  replaced  by  sesquioxide  of  iron 
in  many  varieties  of  mica  (lepidomelane),  and  magnesia  quite  generally 
by  protoxide  of  iron  and  manganese.  The  potassium  in  the  compound 
K  is  in  part  replaced  by  hydrogen,  sodium,  and  lithium.  The 
muscovites  and  phlogopites  are  but  slightly  attacked  by  acids ;  biotites 
are  strongly  attacked  at  high  temperatures. 

Under  the  name  biotite  are  here  included  those  varieties  of  mag- 
nesia mica  which  Tschermak  has  termed  meroxene  and  lepidomelane. 
They  are  isomorphous  mixtures  of  the  molecules  K  and  M,  in  which 
the  molecule  K  generally  predominates.  They  contain  potash  and 
water,  but  only  a  little  soda,  and  scarcely  any  lithia,  They  are  the 
heaviest  micas,  with  a  specific  gravity  from  2.8-3.2,  which  increases 
with  the  percentage  of  iron.  All  biotites  are  micas  of  the  2d 
order  ;  the  axial  plane  lies  in  the  plane  of  symmetry;  the  inclination 
of  the  bisectrix  a  to  the  normal  to  oP  is  generally  very  small,  the  axial 
angle  extremely  variable.  In  the  rock-making  biotites  the  axial  angle 
is  mostly  very  small,  so  that  the  biaxial  character  is  scarcely  determin- 
able.  Nevertheless,  even  here  there  are  values  for  %E  which  exceed 
the  limit  of  56°,  given  by  Tschermak.  The  inclination  of  the  bisectrix 
to  the  normal  to  oP,  which  is  generally  less  than  1°,  occasionally  reaches 
5°  to  8°.  Large  extinction  angles  appear  to  accompany  large  axial 
angles.  Small  axial  angles  occur  both  in  faintly  colored  and  strongly 
colored  biotites ;  large  axial  angles  exclusively  in  strongly  colored  ones. 
The  pleochroism  is  always  strong ;  the  rays  (b  and  c)  vibrating  par- 
allel to  the  cleavage  are  almost  completely  absorbed  in  the  dark- 
colored  biotites,  those  parallel  to  only  slightly.  Differences  between 
the  rays  vibrating  parallel  to  b  and  c  are  more  noticeable  as  the  angle 
^iE  is  greater,  and  sometimes  b,  sometimes  c,  is  more  strongly  absorbed. 
The  absorption  scheme  is  c^b>Ct.  Plates  parallel  to  the  cleavage 
are  dark  brown  or  dark  green  to  opaque,  with  slight  difference  of  color 
when  rotated  over  the  polarizer;  sections  inclined  to  the  cleavage  are 
dark  brown  or  dark  green  when  the  cleavage  cracks  lie  parallel  to  the 
principal  section  of  the  polarizer,  light  yellow  to  red  or  light  green 
when  at  right  angles  to  it.  Similarly  strong  differences  of  absorption 
are  only  exhibited  in  basaltic  hornblendes,  in  tourmaline  and  allanite. 

Biotite  is  equally  common  in  the  massive  rocks  and  in  the  Archaean, 
and  is  one  of  the  most  characteristic  products  of  contact-metamor- 


260          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

phism  in  certain  rocks.  In  the  eruptive  rocks  it  is  one  of  the  oldest 
secretions,  being  formed  immediately  after  the  ores,  zircons,  and  apa- 
tites, which  minerals  are  frequently  included  in  the  biotite.  Fluid 
inclusions  are  found  in  it  everywhere,  but  not  constantly.  They  are 
not  generally  observed  in  the  thin  sections,  as  they  usually  disappear  in 
the  process  of  grinding;  on  the  other  hand,  they  are  found  in  plates 
split  off  from  the  rock  more  frequently  than  would  be  expected,  judg- 
ing from  the  perfect  cleavage  of  the  mineral.  Biotite  twins  occur  in  all 
eruptive  rocks,  which,  however,  are  only  recognized  by  certain  distor- 
tions of  the  interference  figure  in  convergent  light,  and  by  the  pleo- 
chroism  in  sections  inclined  to  the  base.  When  the  interference  figure 
is  distinctly  biaxial,  it  is  often  observed  that  the  axial  rings  are  divided 
into  halves,  which  do  not  exactly  fit  one  another — a  phenomenon  which 
has  been  imitated  by  Bauer  by  placing  on  one  crystal  a  thin  plate 
turned  60°  to  the  first,  or  in  the  position  of  a  twin  along  co^P  (110), 
Since  the  rays  vibrating  parallel  to  b  and  c  are  differently  absorbed, 
and  the  lamellae  of  a  twinned  crystal  are  cut  in  different  directions  by 
the  section,  they  exhibit  different  colors  in  consequence  of  the  pleo- 
chroism,  and  also  different  interference  colors  between  crossed  nicols. 
Cohen  states  that  the  pleochroic  halos  in  biotite  are  dispelled  by 
being  heated  to  redness,  after  the  mineral  has  been  treated  with 
acids. 

The  biotites  of  the  older  granular  massive  rocks  occasionally 
enclose  great  quantities  of  rutile  needles  and  sagenite  webs:  in  many 
cases  these  are  undoubtedly  primary  inclusions ;  in  others  probably 
secondary,  since  they  only  occur  in  the  partly  altered  biotites,  and  are 
wanting  in  the  perfectly  fresh  ones.  This  indicates  the  presence  of 
titanium  in  the  fresh  biotites.  The  primary  as  well  as  the  secondary 
rutile  needles  seldom  lie  irregularly,  but  are  frequently  in  three  sys- 
tems, which  intersect  at  60°,  and  are  parallel  to  the  rays  of  the  pressure 
figure  (PL  XXII.  Fig.  1).  Occasionally,  single  needles  of  rutile  are 
found  parallel  to  the  rays  of  the  percussion  figure,  and  then  almost 
always  parallel  to  the  leading  ray. 

In  the  porphyritic  rocks  biotite  generally  occurs  only  as  one  of  the 
oldest  generations  ;  its  crystallization  occasionally  repeats  itself  in  a 
second  generation  as  a  constituent  of  the  groundmass.  If  the  older 
biotite  has  been  subjected  to  magmatic  corrosion,  it  is  surrounded,  like 
basaltic  hornblende,  with  a  dark  border,  which  consists  of  a  mixture  of 
magnetite  and  augite.  Glass  inclusions  have  not  been  observed  in  the 
biotites. 

The  biotite  of  eruptive  rocks  is  found  regularly  intergrown  with 


THE  MICAS.  261 

basaltic  hornblende  and  augite,  the  basal  plane  of  the  former  coincid- 
ing with  the  cleavage  faces  of  the  latter  minerals,  as  in  granites,  syen- 
ites,  diorites,  trachytes,  anclesites,  etc.  It  occurs  intergrown  with 
muscovite  in  true  granites  only.  The  biotite  of  Archaean  rocks  does 
not  form  crystals,  but  irregularly  bounded  flakes  and  plates,  usually 
-elongated  in  the  direction  of  the  schistosity.  Otherwise  it  possesses 
the  properties  of  the  biotite  in  eruptive  rocks.  Its  frequent  inter- 
growth  with  muscovite  and  paragonite  is  to  be  noted.  Rutile  inter- 
positions also  occur  as  in  the  granular  massive  rocks.  Evidences  of 
chemical  corrosion  are  entirely  absent,  but  those  of  mechanical  defor- 
mation are  wide-spread.  The  color  is  usually  brown  in  the  Archaean 
rocks,  but  green  colors  are  not  uncommon,  especially  when  accom- 
panied by  green  amphibole  ;  green  biotite  never  occurs  in  porphyritic 
massive  rocks,  and  very  rarely  in  the  granular  ones. 

The  biotite,  which  is  one  of  the  most  distinctive  minerals  of  the 
hornstones  in  the  granite  contact,  zones,  forms  round  or  indented 
plates  of  highly  characteristic  chocolate-brown  color;  it  is  green  in 
only  a  few  localities,  and  then  is  generally  weakly  pleochroic.  The 
biotites  are  comparatively  easily  decomposed  minerals.  At  first,  under 
the  action  of  the  natural  reagents,  the  brown  color  is  changed  to  green 
without  affecting  any  other  of  the  optical  properties,  but  the  elasticity 
of  the  plates  disappears.  In  a  more  advanced  stage  the  green  color 
fades  out,  and  the  mica  is  completely  bleached.  This  process,  which 
appears  to  be  a  leeching  out  of  the  iron,  starts  from  the  periphery  and 
proceeds  along  the  cleavage,  often  very  irregularly.  In  other  cases 
biotite  is  altered  into  green  chlorite;  the  strong  double  refraction  de- 
creases rapidly ;  the  distinct  lamellar  structure  gives  place  to  scaly 
fibrous  structure,  combined  with  which  there  is  often  a  fraying  out  of 
the  biotite  (PL  XXII.  Fig.  2).  At  the  same  time  lenticular  masses 
of  the  carbonates  are  deposited  between  the  lamellae,  together  with 
quartz  and  the  iron  ores  ;  or,  in  place  of  the  carbonates  and  quartz, 
epidote  may  occur  under  the  same  conditions  (PI.  XXII.  Fig.  3). 
Upon  the  further  advancement  of  this  process,  the  biotite  may  be 
completely  pseudomorphosed  into  a  mixture  of  carbonates  or  epidote 
with  iron  ores  and  quartz. 

,  Under  anomite  are  here  included  those  rock-making  micas  which 
from  habit  and  color  belong  to  the  biotite  series,  but  which  are  distin- 
guished from  biotite  by  the  fact  that  the  axial  plane  is  not  parallel  to 
the  leading  ray  of  the  percussion  figure — that  is,  does  not  lie  in  the 
plane  of  symmetry,  but  is  normal  to  it.  In  form  and  pleochroism 
they  are  quite  like  the  biotites ;  the  twinning,  which  is  particularly 


262         PHYSIOGRAPHY  OF  TEE  ROCK-MAKING  MINERALS. 

frequent,  is  also  the  same.  The  inclination  of  the  bisectrix  a  to  the 
normal  to  oP  is  generally  greater,  as  Tschermak  found  it  to  be  in  the 
mica  which  he  called  anomite  from  Greenwood  Furnace,  N.  J.,  and 
from  Lake  Baikal,  Siberia.  It  readies  4°,  and  facilitates  the  recognition 
of  the  twinning  in  longitudinal  sections.  The  axial  angle  is  small,  about 
10°,  but  in  many  occurrences  reaches  25°  and  over.  Tschermak  found 
the  dispersion  p  >  v ,  while  for  the  rock-making  anomites  it  is  oftener 
p  <  v .  Evidently  there  are  micas  of  the  second  order  here  called 
anomite,  which  cannot  be  directly  united  with  the  anomite  of  Tscher- 
mak. This  is  in  consequence  of  the  lack  of  chemical  investigation 
upon  the  dark  rock-making  micas  of  the  biotite  series  with  normal 
symmetrical  axial  position. 

F.  Becke*  found  anomite  as  a  constituent  of  a  quartz  diorite 
porphyrite  from  Steinegg,  in  Lower  Austria,  in  zonally  built  crystals  ; 
the  light  greenish  brown  centre  is  surrounded  by  a  dark  brown  shell 
which  behaves  uniaxially.  According  to  this,  the  negative  axial  angle 
in  anomite  decreases  with  the  percentage  of  iron,  which  Tschermak 
found  in  the  occurrences  cited.  Beckef  also  found  it  as  a  secondary 
mineral  in  an  altered  olivine  rock  occurring  as  an  intercalated  mass  in 
the  diorite  schist  at  Diirnstein,  Lower  Austria.  The  color  is  reddish 
brown,  2^  =  18°  54/.  Eichstadt^:  recognized  it  in  great  plates  in  a 
mellilite  basalt  from  Alno,  described  by  Tornebohm  ;  this  is  also 
reddish  brown,  2J57  —  8°-10°.  G.  Lattermann  found  anomite  and 
biotite  associated  with  one  another  in  the  same  rock  :  for  example,  in 
kersantite  from  Michaelstein,  near  Blankenburg,  in  the  liartz  Mts. 

—  10°-22°) ;   in   mica  andesite   from   Eepistye,  near   Schemnitz. 

—  10°-4:00);  in  the  nepheline  rocks  of  Katzenbuckel,  near  Heidel- 
berg (2^=40°  about).     The  dispersion  is  that  of  biotite  p  <  v\  the 
absorption  sometimes  b  >  C,  sometimes  c  >  ft,  always  a  <  C  and  b.  The 
color   of    rock-making   anomite  is    always  brown    or   reddish  brown, 
never  true  green.     Moreover,  the   anomites  of  rocks  are  somewhat 
brittle. 

Rubellan  is  the  name  applied  by  Breithaupt  to  reddish  or  rust- 
brown  volcanic  biotite,  more  or  less  impregnated  with  iron  ochre  and 
specular  iron,  which  occur  like  inclusions  in  the  tuffs  and  lavas  of  Lake 
Laach  and  of  the  Bohemian  "  Mittelgebirge"  The  name  has  been 
subsequently  applied  to  similar  biotites  of  older  eruptive  rocks  which 
in  part  are  colored  by  the  secretion  of  iron  oxide,  and  are  no  longer 

*  T.  M.  P.  M.  1882.  IV.  151. 
f  T.  M.  P.  M.  1883.  V.  332. 
Geol.  Foren.  i.  Stockh.  Fordhl.  1884.  VII.  No.  87.  194. 


THE  MICAS.  263 

elastic.  The  inner  lamellae  of  rubellan  often  possess4  the  character- 
istics of  unaltered  ferruginous  biotites.  The  rubellans  of  Lake  Laach 
and  Schima  have  been  investigated  microscopically  and  chemically  by 
M.  U.  Hollrung.* 

The  phlogopite  series  includes  phlogopite  and  zinnwaldite.  Both 
occur  to  a  limited  extent  as  rock  constituents.  Both  are  mica  of  the 
second  order ;  the  negative  bisectrix  is  noticeably  inclined  (2^°-4°)  to 
the  base ;  they  consist  of  isomorphous  mixtures  of  the  molecules 
K,  M,  S,  S'.B 

Phlogopite  is  chiefly  confined  to  the  granular  limestones  of  the 
Archaean,  in  which  it  forms  crystals  and  plates.  It  becomes  trans- 
parent and  colorless  to  yellowish  or  greenish,  seldom  brownish  yellow; 
its  pleochroism  is  slight,  the  absorption  c  >  b  >  a.  Dispersion  p  <  v , 
as  in  biotite. 

The  abundance  of  inclusions  in  the  large  phlogopites  of  the  Cana- 
dian occurrences  is  well  known,  as  well  as  in  those  of  the  United 
States.  Besides  quartz  and  garnet  in  quite  flat  tablets,  specular  iron 
is  particularly  frequent  in  opaque  or  red  to  yellow  and  grayish-yellow 
transparent  plates,  elongated  and  arranged  parallel  to  the  rays  of  the 
pressure  figure.  In  other  localities  it  carries  tourmaline  crystals  be- 
tween its  lamellae,  which  lie  in  rows  intersecting  at  60°,  and  are  so 
thin  that  they  glisten  with  the  most  brilliant  Newton  colors.  They 
give  rise  to  a  distinct  asterism.  Rutile  occurs  in  place  of,  or  associated 
with,  tourmaline,  in  the  same  manner  and  with  the  same  effect.  The 
same  minerals  also  occur  in  three  less  pronounced  systems  parallel  to 
the  rays  of  the  percussion  figure,  giving  rise  to  three  more  rays  of 
light,  and  producing  a  six-rayed  asterism. 

Phlogopite  alters  into  fibrous  scaly  masses,  which  appear  to  con- 
sist chiefly  of  talc ;  rutile,  also,  not  infrequently  occurs  as  a  secon- 
dary mineral,  and  indicates  that  the  mica  originally  contained  titanic 
acid. 

Zinnwaldite  or  lithionite  here  includes  fluorine-bearing  lithia-iron 
micas,  which  appear  to  be  isomorphous  mixtures  of  the  molecules  K, 
M,  S  and  S'  in  very  different  proportions,  and  whose  color  therefore 
changes  from  dark  brown  by  transmitted  light  to  light  yellow  and  gray- 
ish white.  The  axial  plane  lies  in  the  plane  of  symmetry,  that  is,  parallel 
to  the  leading  ray ;  the  axial  angle  diminishes  from  about  60°  to  10°, 
and  becomes  smaller  as  the  color  becomes  darker,  that  is,  as  the  iron 
percentage  rises.  In  the  light  yellow  varieties  the  inclination  of  the 

*  T.  M.  P.  M.  1883.  V.  304-331. 


264         PHYSIOGRAPHY  OF  THE  IWCK-MAKING  MINERALS. 

negative  bisectrix  to  the  normal  to  oP  is  distinctly  noticeable ;  it  ap- 
pears to  be  very  small  in  the  dark  colored  varieties.  The  dispersion  is 
weak,  p  >  v.  The  pieochroism  varies  between  dark  brown  for  c  and 
b,  yellowish  brown  to  reddish  for  a,  in  the  dark  varieties;  brownish  gray 
for  c  and  b  and  almost  colorless  for  ft  in  the  light  varieties.  The  ab- 
sorption is  always  distinct,  c  >  b  >  ft.  The  presence  of  lithia,  recog- 
nized in  the  flame,  easily  separates  this  mica  from  all  other  micas  of  the 
2d  order,  and  the  position  of  the  axial  plane  distinguishes  it  from 
lepidolite. 

The  specific  gravity  varies  greatly  with  the  iron  percentage,  and 
rises  to  3.2.  Sandberger  *  describes  its  occurrence  in  the  tin-bearing 
granites  of  the  Erzgebirge,  Fichtelgebirge,  Central  France,  and  Corn- 
wall. Topaz  occurs  in  the  same  granites,  and  rutile  and  cassiterite 
form  microscopic  inclusions  in  these  lithionites,  with  pleochroic  halos, 
not  infrequently  with  zircon  and  topaz.  It  also  occurs  in  the  pegmatitic 
secretions  of  granite  and  gneiss,  when  it  is  usually  peachblow-red  by 
incident  light  and  colorless  by  transmitted  light. 

The  micas  of  the  tnuscomte  series,  which  occur  as  rock  constituents, 
are  always  light  colored,  and  do  not  form  regularly  bounded  crystals, 
but  lamellar  individuals  and  aggregates.  They  are  all  micas  of  the 
first  order,  the  axial  plane  lies  normal  to  the  plane  of  symmetry  and 
to  the  leading  ray.  The  dispersion  is  p  >  v .  The  specific  gravity  is 
2.83  -  2.9.  They  are  mostly  elastic,  seldom  brittle. 

Lepidolite,  essentially  *3[3(Li,H)2O,  3A12O3,  6S1OJ  +  Si10O8Fl24, 
is  generally  pale  peachblow-red  in  thick  plates  by  incident  light,  but 
becomes  transparent  and  colorless  in  very  thin  plates.  Pieochroism  is 
not  noticeable,  yet  it  can  be  seen  that  in  longitudinal  sections  rays  vi- 
brating in  the  cleavage  plane  are  more  strongly  absorbed  than  those 
normal  to  it.  The  bisectrix  stands  apparently  normal  to  oP  (001);  the 
axial  angle  is  large,  50°-70°.  It  is  said  to  accompany  rnuscovite  in 
some  granites  (Elba,  Schaistansk)  and  in  the  pegmatitic  secretions  of 
many  granites  and  gneisses,  and  is  only  distinguished  from  this  with 
certainty  by  the  reaction  for  lithia. 

Muscovite,  K2O,  2H2O,  3A12O8,  6SiO2,  is  wholly  foreign  to  the 
massive  rocks,  with  the  exception  of  the  granites  and  quite  isolated 
quartz  porphyries ;  it  is  not  a  volcanic  mineral.  On  the  other  hand, 
potash  mica  plays  a  prominent  role  in  the  Archaean  rocks  and  the  re- 
gionally metamorphosed  members  of  the  sedimentary  formations.  The 
large  tabular  occurrences  in  the  granites,  gneisses,  and  mica  schists  are 


*L.  J.  1885.11. 


THE  MICAS.  265 

known  as  muscovite  proper,  in  distinction  to  the  more  microscopic  or 
finely  lamellar  to  scaly  and  dense  sericites  of  the  phyllites,  porphyroids, 
and  clay  slates.  The  muscovites  are  transparent  and  colorless  to  light 
greenish  or  light  yellowish,  occasionally  colored  red  by  flakes  of  hematite; 
they  are  without  actual  pleochroism,  but  with  recognizable  absorption 
of  the  rays  vibrating  parallel  to  the  cleavage.  They  are  well  charac- 
terized by  strong  negative  double  refraction,  by  the  brightly  colored 
interference  figure  of  cleavage  plates,  by  the  large  axial  angle,  40°- 70°, 
and  by  the  imperceptible  inclination  of  the  extinction  to  the  cleavage. 
The  large  axial  angle  distinguishes  them  from  talc.  Pleochroic  halos  are 
very  common.  The  specific  gravity  facilitates  their  mechanical  separa- 
tion from  the  biotites  and  zinnwaldites.  The  freshness  of  muscovite 
is  very  characteristic ;  it  does  not  appear  to  suffer  from  the  action  of  the 
atmosphere. 

S&ridte,  like  muscovite,  forms  irregularly  bounded  plates  of  very 
small  thickness,  which  are  usually  drawn  out  into  long  narrow  stripes. 
In  consequence  of  their  geological  position  they  often  appear  twisted 
and  bent,  or  the  plates  are  arranged  spirally  arid  like  rosettes  about 
a  longitudinal  axis.  All  sections  through  such  aggregates  show  the 
cleavage  cracks  slightly  curved  and  not  straight,  giving  the  impression 
that  the  structure  is  a  fibrous  and  not  a  lamellar  one ;  indeed,  with 
strongly  crumpled  and  rolled  out  sericite-bearing  rocks  the  structure 
appears  to  be  a  felty  fibrous  one,  although  it  is  scaly  and  lamellar. 

The  optical  behavior  is  exactly  the  same  as  that  of  muscovite,  but 
strikingly  small  axial  angles  (25°- 30°)  are  often  observed  in  the  seri- 
cites of  phyllites.  It  is  probable  that  substances  of  different  composi- 
tion are  included  under  sericite.  The  distinction  from  talc  is  only 
possible  through  chemical  reaction,  treatment  with  cobalt  solution,  or 
better,  with  hydrofluosilicic  acid. 

Damourite  is  small-leaved  muscovite,  which,  like  sericite,  can  assume 
a  talc-like  habit.  The  muscovites  together  with  feldspar  are  the  most 
characteristic  minerals  of  dynamo-metamorphic  origin,  and  arise  from 
true  sedimentary  rocks  (clay  slates  and  grauwacke  schists),  and  from 
eruptive  rocks  and  their  tuffs.  They  are  also  formed  by  the  processes 
which  deposit  ores  along  the  cracks  and  fissures  of  faulted  Archaean 
masses. 

The  manifold  nature  of  the  occurrence  of  muscovite  is  shown  in  the 
dissemination  of  this  mineral  as  pseudomorphs  after  other  silicates,  such 
as  feldspar,  nepheline,  leucite,  andalusite,  cordierite,  beryl,  etc. 

Paragonite,  Na2O,  2H2O,  3A12O8,  6SiO2,  has  not  yet  been  ob- 
served in  eruptive  rocks.  It  is  confined  to  crystalline  schists  and  phyl- 


266          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

lites.  Like  mnscovite,  it  forms  irregularly  bounded  plates,  on  which 
indications  of  a  six-sided  outline  have  only  occasionally  been  noticed, 
and  which  yield  narrow,  lath-shaped  longitudinal  sections,  whose  longer 
sides  are  parallel  to  the  perfect  cleavage.  By  transmitted  light  color- 
less ;  axial  angle  large  to  very  large.  Dispersion  and  absorption  as  in 
muscovite.  Paragonite  also  sinks  to  fine  scaly  aggregates,  possessing 
a  dense  talc-like  appearance,  and  presenting  a  sericitic  modification. 
The  microscopical  distinction  from  talc  lies  in  the  large  axial  angle.  It 
can  only  be  distinguished  from  muscovite,  chemically,  by  treatment 
with  hydrofluosilicic  acid,  by  which  process  hexagonal  crystals  of  sodium 
fluosilicate  are  almost  exclusively  obtained.  Paragonite  schists  often 
contain  garnet,  staurolite,  disthene,  tourmaline,  rutile,  actinolite,  mag- 
nesite,  and  dolomite  in  beautiful  crystals. 


The  Ottrelite  Group. 
Literature. 

C.  BARROIS,  Note  sur  le  chloritoide  du  Morbihan.    Bull.  Soc.  Min.  Fr.  1884.    VII. 
37-43. 

F.  BECKE,  Gesteine  der  Halbinsel  Chalcidice.     T.  M.  P.  M.  1878.  I.  269-272. 

A.  DBS  CLOIZEATTX,  Sur  la  forme  cristalline  et  les  caracteres  optiques  de  la  Sismon- 

dine.     Bull.  Soc.  min.  Fr.  1884.  VII.  80-85. 
H.   VON  FOULLON,  Ueber  die  petrographische  Beschaffenheit  der  krystallinischen 

Schiefer  der  untercarbonischen  Schichten  und  einiger  iilterer  Gesteine  aus  der 

Gegend  von  Kaisersberg  bei  St.  Michael  ob  Leoben.    Jahrb.  k.  k.  geol.  Reichs- 

anst.  1883.  XXXIII.  220  sqq. 
A.  VON  LASAULX,  Ueber  Glaukophangesteine  der  He  de  Groix.     Sitzungsber.  d.  nie- 

derrhein.  Ges.  in  Bonn.  3.  Dec.  1883. 
A.  RENARD  et  CH.  DE  LA  VALLEE-POUSSIN,  Note  sur  1'Ottrelite.     Annales  de  la  Soc. 

geol.  de  Belgique.  1879.  VI.  51-68. 

G.  TSCHEBMAK  und  L.  SIPOCZ,  Die  Clintonitgruppe.    S.  W.  A.  1878.  LXXVIII.  Nov. 

Under  the  ottrelite  group  are  here  included  the  very  closely  re- 
lated minerals  called  ottrelite,  chloritoid,  chlorite  spar,  masonite,  and  sis- 
mondine.  The  name  ottrelite  is  chosen  for  that  of  the  group  because 
it  immediately  suggests  the  geologically  characteristic  position  of  these 
minerals.  The  statements  of  the  above-cited  authors  regarding  these 
minerals  differ  very  widely,  and  cannot  be  altogether  reconciled.  The 
following  data,  which  agree  with  the  observations  of  Tschermak,  except 
in  a  difference  with  regard  to  the  pleochroism,  are  derived  from  the 
study  of  ottrelite  from  Serravezza,  from  Ottre  and  St.  Hubert  in  the 
Ardennes  Mountains,  of  sismondine  from  Pregratten,  Tyrol,  and  St. 
Marcel,  Piedmont,  of  chloritoid  from  Kossoibrod,  Urals,  Harvey  Hills 


OTTRELITE  GROUP.  267 

near  Leeds  and  Inverness  in  Canada,  and  of   masonite   from  Natic 
Rhode  Island. 

Rock-making  ottrelite  forms  single  crystals,  generally  with  quite 
incomplete  boundaries;  also  disk-like  to  lenticular  or  spindle-shaped 
grains,  or  somewhat  larger  lamellar  masses,  up  to  3  c.m.  in  diameter ; 
besides  sheaf-shaped  and  tuft-like  or  irregular  aggregates  of  crystals  and 
crystal  grains.  They  always  lie  irregularly  scattered  in  the  rocks,  quite 
like  chiastolites  in  the  hornstones.  Whenever  a  crystal  form  is  ob- 
served, it  is  that  of  a  thin,  micaceous  hexagonal  pkte  (PI.  XXII.  Fig. 
.  4),  whose  plane  angles  appear  to  be  120°.  One  pair  of  faces  may  dis- 
appear,' and  rounding  and  mechanical  deformation  produce  all  theinter- 
termediate  stages  to  grains  of  the  most  different  shape.  Hence  there 
arise  lateral  boundary  lines,  as  with  the  feldspars  of  the  rhombic  por- 
phyries, which  do  not  intersect  at  120°,  but  at  any  angle  whatever. 
The  form  will  here  be  given  as  oP  (001),  the  tabular  face,  +  P  (111), 
and  GO  Poo  (010).  The  base  glistens  strongly,  but  is  generally  scaly,  and 
broken  up  into  small  areas  ;  it  is  also  crooked  and  bent.  The  lateral 
faces  are  dull,  and  have  a  resinous  lustre,  and  occasionally  are  notice- 
ably furrowed  parallel  to  the  base.  Sections  parallel  to  the  base  are 
hexagonal,  rhombic,  or  irregular  ;  all  other  sections  are  lath-shaped.  Ap- 
parently simple  crystals  almost  always  show  themselves  optically  as 
polysynthetic  twins,  in  which  the  individuals  are  in  contact  along  their 
bases  (PI.  XXII.  Fig.  5),  but  are  so  placed  Jthat  each  is  turned  120°  with 
respect  to  the  adjacent  ones.  The  twinning  law  corresponds  exactly  to 
that  of  mica.  Less  frequently  the  individuals  are  in  contact  along  a 
lateral  face,  whose  projection  on  the  base  is  parallel  to  the  trace  of  P 
(111).  Hence  the  twinning  is  generally  not  noticeable  on  the  base. 
More  frequently  the  composition  plane  is  irregular,  and  the  individuals 
cross  one  another  in  hour-glass-like  faces  (PL  XXII.  Fig.  6). 

A  very  regular  zonal  structure  in  concentric  hexagons  is  common, 
especially  in  the  fine  Canadian  chloritoids. 

The  ottrelites  possess  a  good  cleavage  parallel  to  the  base,  which 
always  shows  itself  in  numerous  sharp  cracks  in  thin  sections,  when 
the  plates  are  thin  enough  and  are  well  ground.  They  are  wanting  in 
thicker  plates,  and  in  those  which  are  so  thin  that  they  lie  in  the  sec- 
tion as  whole  bodies.  The  cleavage  is  not  so  perfect  as  that  of  mica 
and  hornblende ;  it  somewhat  resembles  that  of  augite.  The  cleavage 
plates  are  extremely  brittle,  and  are  only  transparent  when  very  thin. 
Besides  the  basal  cleavage,  there  is  in  many  ottrelites  another  parallel 
to  two  lateral  faces,  whose  traces  on  <?_P(001)  intersect  at  120°.  The 
corresponding  cracks  are  less  numerous,  are  often  interrupted  or  pass 


268          PHYSIOGRAPHY  OF  TUB  ROCK-MAKING  MINERALS. 

into  irregular  fractures.  The  large-leaved  ottrelites  (St.  Hubert,  Inve_ 
ness)  exhibit  them  well ;  the  rounder  to  spinel-shaped  ones,  badly  or  not 
at  all.  Finally,  there  is  quite  an  imperfect  parting  parallel  to  the  plane 
of  symmetry,  whose  cracks  bisect  the  obtuse  angle  of  the  cleavage  just 
described.  All  of  these  lateral  cleavages  are  completely  obscured  in 
many  occurrences  by  irregular  cracks,  which  evidently  correspond  to 
an  internal  fracturing  due  to  mountain  pressure. 

The  twinning,  fracturing,  brittleness,  and  deep  color  of  the  ottre- 
lites place  great  obstacles  in  the  way  of  their  optical  investigation. 
The  sections  must  be  very  thin  in  order  to  observe  simple  individuals 
and  not  twinned  ones.  The  ottrelites  become  transparent,  and,  accord- 
ing to  the  position  of  the  section,  green  or  blue,  or  even  colorless. 
Their  index  of  refraction  is  not  inconsiderable;  according  to  Glad- 
stone's law,  n  =  1.718.  Hence  the  rough  surface  of  the  sections  in 
Canada  balsam.  The  double  refraction  is  weak  even  parallel  to  the  axial 
plane,  and  the  interference  colors  in  sections  which  are  not  very  thin  do 
not  exceed  those  of  the  1st  order — a  good  means  of  distinction  from 
mica.  The  extinction  in  basal  sections  lies  parallel  to  one  edge  of  the  hex- 
agon, or  to  the  diagonals  of  the  cleavage  cracks,  which  intersect  at  120°. 
In  sections  inclined  to  the  principal  cleavage  the  extinction  sometimes 
lies  parallel  to  the  basal  cleavage ;  the  sections  are  then  from  the  ortho- 
diagonal  zone  ;  at  other  times  the  extinction  takes  place  at  various  angles 
to  the  principal  cleavage.  The  extinction  angle  in  sections  parallel  to 
oo .Poo  (010)  is  from  12°-1S°  in  different  occurrences.  The  successive 
twin  lamellae  of  these  sections  almost  never  show  equal  extinction 
angles  measured  from  the  trace  of  tne  composition  plane,  which  proves 
that  the  twinning  plane  is  not  normal  to  the  plane  of  symmetry.  The 
angle  between  the  extinctions  in  two  adjacent  twin  lamellae  may 
reach  40°. 

In  convergent  light  a  positive  bisectrix  emerges  obliquely  from  the 
principal  cleavage  face.  The  axial  angle  must  vary  considerably,  since 
in  many  occurrences  an  axis  appears  on  the  edge  of  the  field  of  view 
and  the  dispersion  p  >  v  can  be  observed,  while  in  most  cases  the  axes 
are  not  visible.  The  axial  plane  is  parallel  to  the  plane  of  symmetry ; 
it  bisects  the  obtuse  angle  of  the  prismatic  cleavage. 

All  minerals  of  the  ottrelite  group,  except  the  Styrian  occurrences 
described  by  Foullon,  are  remarkable  for  a  highly  characteristic  pleo- 
chroism,  which  is  of  great  diagnostic  importance.  In  basal  sections 
the  ray  vibrating  parallel  to  the  axial  plane  is  olive-green,  that  normal 
to  it  plum-blue  to  indigo-blue ;  in  sections  inclined  to  the  cleavage  the 
ray  vibrating  nearly  normal  to  the  cleavage  is  yellowish  green,  that  al- 


EPIDOTE.  269 

most  parallel  to  it  either  blue  or  olive-green.  Therefore  c  =  yellowish 
green,  b  =  plum-blue  to  indigo-blue,  d  —  olive-green. 

H.  =  6-7.  Sp.  gr.  =  3.53-3.55.  The  chemical  composition  is  not 
definitely  known,  because  of  the  difficulty  of  removing  the  abundant 
interpositions.  Tschermak  has  given  the  formula  for  the  purest 
chlorite  spar  as  probably  H2O,  FeO,  A12O3,  SiO2.  In  this  a  variable 
portion  of  FeO  is  replaced  by  MgO ;  in  the  ottrelite  from  Ottre  a  con- 
siderable amount  is  replaced  by  MnO.  Ordinary  acids  do  not  attack 
the  minerals  of  the  ottrelite  group.  Fused  with  caustic  potash,  cleavage 
plates  yield  etched  figures  on  the  basal  plane,  with  apparently  triangu- 
lar or  hexagonal  outline ;  they  are,  in  fact,  monosymmetric,  and  their 
plane  of  symmetry  coincides  with  the  plane  of  the  optic  axes  (Sanger). 

The  rock-making  ottrelite  minerals  generally  contain  a  great  many 
different  interpositions,  among  which  are  quartz  grains,  ores,  carbona- 
ceous particles,  rutile  needles,  and  tourmaline  columns.  The  arrange- 
ment of  these  inclusions  is  irregular. 

The  ottrelite  minerals  are  almost  exclusively  confined  to  phyllitic 
schists  and  indicate  dynamo-metamorphic  processes.  Such  schists  are 
found  in  the  Ardennes,  the  Pyrenees,  the  Apennines,  in  Styria,  and 
are  particularly  fine  in  the  Province  of  Quebec,  Canada,  and  in  Rhode 
Island.  Sismondine  occurs  with  glaucophane  at  Zermatt,  Switzerland, 
in  Val  Chisone  in  Piedmont,  and  on  the  lie  de  Groix  in  Brittany. 


Epidote. 
Literature. 

C.  KLEIN,  Die  optischen  Eigenschaften  des  Sulzbacher  Epidot.    N.  J.  B.  1874. 
1-21. 

Rock-making  epidote  seldom  possesses  sharp  crystal  forms,  and  then 
most  frequently  shows  the  faces  M  =  oP  (001),  T—  ooPdo  (100),  r  = 
Poo  (101),  in  the  orthodiagonal  zone.  The  angles  at  which  these  faces 
intersect  are  M^T=  115°  24',  M/\r  =  116°  IS',  T/\r  =  128°  18'. 
The  crystals  are  always  more  or  less  elongated  parallel  to  the  axis  of 
symmetry.  The  faces  cutting  this  angle  are  frequently  undeveloped. 
Therefore  sections  parallel  to  the  axis  b  are  long  lath-shaped,  those 
parallel  to  the  plane  of  symmetry  approximately  hexagonal  (Fig.  93). 
Moreover,  the  face  Tis  generally  much  smaller  than  r;  it  is  rarely  the 
reverse.  T  may  also  be  wanting,  and  sections  parallel  to  ooPoo  (010) 
are  then  rhombic.  More  frequently  a  crystallographic  boundary  is 
entirely  wanting,  and  the  epidote  forms  columns  parallel  to  £,  or 


270         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


irregularly  angular  individuals  and  aggregates  without  regular  bound- 
aries to  their  cross-sections. 

Twinning  is,  on  the  whole,  rare  in  rock-making  epidote ;  in  many 
rocks,  however,  it  is  very  abundant.  The  twinning  and  composition 
plane  is  then  T  (Fig.  .94).  Between  the  two  larger  twins  there  are 
occasionally  several  delicate  twin  lamellae. 

The  perfect  cleavage  parallel  to  M  (001)  shows  itself  in  sharp 
tracks,  which,  however,  are  not  so  numerous  as  one  would  expect  from 
the  perfection  of  the  cleavage ;  the  cleavage  parallel  to  T(100)  is  repre- 
sented by  but  few  cracks  (PL  XL  Fig.  2).  The  angle  between  these 


cleavage  cracks  varies  from  115°  24'  in  sections  parallel  to  ooPoo  (010) 
to  180°  in  those  from  the  orthodiagonal  zone. 

Rock-making  epidote  becomes  transparent,  and  almost  colorless  or 
pale  yellow,  rarely  yellowish  brown,  pale  green,  very  seldom  redo  The 
index  of  refraction  and  double  refraction  are  very  considerable ;  hence 
the  marginal  total  reflection  and  the  unevenness  of  the  surface  are 
strongly  marked.  The  height  of  the  interference  colors  in  sections 
parallel  to  the  plane  of  symmetry  is  greater  than  for  any  other  silicate, 
being  only  second  to  those  of  rutile,  anatase,  zircon,,  and  the  rhombo- 
hedral  carbonates;  even  in  very  thin  sections  they  are  of  the  3d 
order.  Klein  determined  on  the  epidote  of  Knappenwand  aft  =  1.730, 
j3p  —  1.754,  yp  —  1.768.  Hence  y  —  a  —  0.038.  Michel-Levy  meas- 
ured on  epidote  from  the  same  locality,  y  —  a  =  0.047,  and  on  that  of 
the  ophite  from  Lherz,  y  —  a  =  0.0545  ;  on  that  from  the  schists  of 
He  de  Groix,  y  —  a  —  0.056. 

The  plane  of  the  optic  axes  lies  in  the  clinopinacoid  ;  the  axial  angle 
2HP  =  91°  20',  2  Vp  ==  73°  40';  the  dispersion  is  distinctly  inclined  and 
weak,  p  >  v .  The  negative  first  bisectrix  is  inclined  2°-3°  to  the  ver- 
tical axis,  and  lies  in  the  acute  angle.  Hence  the  scheme,  Fig.  93. 
Cleavage  plates  parallel  to  M  exhibit  an  axis  in  the  margin  of  the  field 
of  view,  whose  hyperbola  in  the  diagonal  position  is  green  on  the  inner 


EPIDOTE. 

W' 

border  and  red  on  the  outer  one ;  isolated  crystals  which  lie  on  r  show 
an  axis  normal  to  r,  the  borders  of  whose  hyperbola  are  red  on  the 
inside  and  green  on  the  outside.  All  sections  from  the  orthodiagonal 
zone  exhibit  in  convergent  light  axial  bars,  axial  figures  or  the  loci  of 
bisectrices,  which  show  that  the  axial  plane  is  normal  to  the  cleavage — 
the  surest  means  of  distinction  from  augite,  with  which  mineral  epi- 
dote may  be  confounded. 

In  parallel  light  sections  from  the  orthodiagonal  zone  extinguish 
parallel  and  normal  to  the  cleavage.  In  sections  from  this  zone  twins 
cannot  be  recognized  by  the  extinction  in  parallel  polarized  light.  In 
the  zone  ooPoo  :  oojPoo  the  angle  between  the  extinction  and  cleavage 
cracks  increases  from  0°  on  T  to  about  28°  on  ooPco  (010).  In  the 
zone  oP :  oojPoo  the  extinctions  lie  between  0°  and  28°. 

The  strong  pleochroism  of  the  Sulzbach  epidotes  disappears  entirely 
in  the  colorless  occurrences  in  rocks,  and  is  faint  in  the  light  colored 
ones.  Thus  in  the  Sulzbach  crystals  a  =  yellow,  b  =  brown,  c  =  green, 
and  6  >  C>  ft;  while  in  the  rock- making  ones  a  =  colorless  to  light 
yellowish  green,  b  =  yellowish  green  to  colorless,  c  =  siskin-green  to 
green  or  light  yellowish  brown.  Absorption  c  >  b  >  a.  Though  the 
difference  of  color  is  so  slight,  yet  the  change  from  green  and  siskin- 
green  to  colorless  or  light  yellowish  in  very  light  colored  epidotes  is 
very  characteristic ;  in  the  uncommon  red  epidotes  the  colors  change 
between  red,  yellow,  and  colorless. 

H.  =  6.5.  Sp.gr.  — 3.3-3.5,  increasing  with  the  percentage  of  iron. 
Chemical  composition,  H2O,  4CaO,  3(Al2Fea)O3,  6SiO2.  It  is  not  at- 
tacked by  acids,  but  is  decomposed  in  HC1,  after  being  heated  to  redness. 

There  is  no  constant  microstructure.  It  is  usually  free  from  in- 
clusions ;  fluid  inclusions  are  more  frequent  than  particles  of  ore  and 
carbonaceous  matter. 

Epidote  never  occurs  as  a  primary  constituent  in  eruptive  rocks  nor 
in  true  Archaean  rocks.  Still,  it  is  a  characteristic  constituent  of  those 
stratified  rocks  (garnet  rocks  and  certain  amphibolites)  which  are  the 
equivalent  of  granular  limestones,  of  paragonite  and  glaucophane 
schists,  of  gneisses  in  the  phyllitic  schists,  and  of  metamorphic  gneisses, 
of  phyllites,  and  green  schists.  It  is  also  one  of  the  commonest  forma- 
tions in  the  lime-silicate  hornstones.  As  a  product  of  weathering,  epi- 
dote is  the  most  frequent  of  all  silicates ;  thus  it  is  formed  in  the  acid 
and  basic  rocks  from  the  feldspars  under  the  influence  of  solutions 
derived  from  the  micas  and  bisilicates.  Saussurite  consists  chiefly  of 
epidote.  Whenever  calcareous  iron  and  magnesia  silicates  chloritize, 
epidote  is  a  constant  side  product,  the  lime  being  deposited  or  removed 


272          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

as  a  carbonate.     Thus  it  is  found  accompanying  the  atmospheric  de- 
composition of  pyroxene,  amphibole,  mica,  and  garnet. 

Allanite.* 
Literature. 

J.  P.  IDDINGS  and  WHITMAN  CROSS,  Widespread  occurrence  of  Allanite   as  an 

accessory    constituent    of    many  rocks.       Am.    Journ.    Sci.    Aug.    1885.  Vol. 

XXX.  108. 
A.  MICHEL-LEVY  and  LACROIX,  Note  sur  un  gisement  fran9ais  d'allanite.      Bull. 

Soc.  Min.  Fr.  1888.  Feb.  Vol.  XI.  No.  2.  65. 
A.    SJOGREN,  Om  Gadolinitens,  orthitens,   samt  med  dessa    likartade  mineraliers 

forhallande  under  mikroskopet.     Geol.  Foren.  i  Stockholm  Forliandl.  1876.  III. 

No.  37.  258. 
A.  E.  TORNEBOHM,  Under  Vega-Expeditionen  insamlade  bergarter.      Vega-Eped. 

vetensk.  jakttagelser.  VI.  Stockholm.  1884.  124. 

Allanite,  which  is  isomorphous  with  epidote,  occurs  as  an  accessory 
constituent  of  many  granites,  diorites,  and  other  rocks ;  in  the  tonalite 
of  Adamello,  according  to  G.  vom  Rath,  f  it  is  often  so  abundant 
as  almost  to  become  an  essential  ingredient.  It  frequently  forms 
completely  bounded  crystals  with  the  faces  aP(OOl),  oo.Po5  (100)  well 
developed,  besides  ccP  (110)  and  jPoo  (Oil),  and  sometimes  two 
orthodomes.  (110)  A  (HO)  =  117°  and  (110)  A  (100)  =  125°,  approxi- 
mately. The  crystals  are  elongated  in  the  direction  of  the  orthoaxis 
J,  as  in  epidote,  and  the  sections  have  similar  shapes.  It  also  occurs  as 
irregular  grains.  Twinning  along  the  plane  oo  P^  (100)  is  frequent. 

The  cleavages  parallel  to  ooP(llO),  ooPo5  (100),  oo Poo  (010),  and 
also  to  0,P(001),  are  occasionally  indicated  by  irregular  cracks,  but 
in  many  occurrences  they  are  entirely  absent. 

Allanite  becomes  transparent  in  thin  sections  with  reddish  brown 
or  greenish  brown  colors.  It  usually  exhibits  a  strong  pleochroism 
from  light  yellowish  or  greenish  brown  to  dark  chestnut-brown.  In 
the  allanite  of  the  granite  from  Font-Paul,  Finisterre,  Michel-Levy 
and  Lacroix  found  d  —  greenish  brown,  b  =  reddish  brown,  and 
C •=  yellowish  brown.  The  mean  index  of  refraction  exceeds  1.78. 
The  double  refraction  is  variable :  in  the  allanite  from  Font-Paul  it  is 
very  feeble,  but  in  that  from  Edenville,  !N".  Y.,  y  —  a  =  0.032, 
according  to  Michel-Levy.  Many  allanites  are  isotropic,  without  show- 
ing any  change  of  form  or  noticeable  signs  of  decomposition. 

*  Expanded  by  the  translator, 
f  Z.  D.  G.  G.  1864.  XIV.  255. 


ALLANITE.  273 

The  plane  of  the  optic  axis  lies  in  the  plane  of  symmetry,  oo  P^ . 
The  axes  of  greatest  and  least  elasticity  bisect  the  angles  between  the 
vertical  axis  6  and  the  clinoaxis  a ;  the  acute  bisectrix  is  a,  and  lies  in 
the  obtuse  angle  between  c  and  a.  The  optical  character,  therefore,  is 
negative,  2  F—  65°  to  70°.  Tlie  optic  axes  are  nearly  normal  to  the 
faces  0P(001)  and  ooPab  (100).  There  is  a  large  dispersion  of  the 
axes  of  elasticity,  which  causes  confused  extinctions  in  parallel 
polarized  light. 

In  many  occurrences,  especially  in  the  granites  and  gneisses,  allanite 
possesses  a  marked  zonal  structure,  accompanied  by  variations  in  the 
directions  of  extinction  and  in  the  color.  In  the  porphyrites,  por- 
phyries, and  volcanic  rocks  zonal  structure  is  almost  entirely  want- 
ing, and  the  color  is  dark  reddish  brown. 

H.  =  5.5-6.  Sp.  gr.  =  3.0-4.2.  Chemical  composition  similar  to 
that  of  epidote,  except  that  part  of  the  Ca  is  replaced  by  Fe,  and  the 
Al  is  largely  replaced  by  the  rare  earths,  Ce,  La,  Di,  Y,  Er.  It  is 
decomposed  by  boiling  hydrochloric  acid. 

Allanite  occurs  as  a  primary  accessory  ingredient  of  many  eruptive 
rocks.  In  the  granite  from  Font-Paul  it  is  one  of  the  oldest  constitu- 
ents, and  is  enclosed  in  biotite  and  surrounded  by  pleochroic  halos. 
It  has  a  wide  distribution  through  a  great  variety  of  rocks  in  the 
United  States,  having  been  found  in  gneiss,  granite,  granite  porphyry, 
quartz  porphyry,  diorite  porphyrite,  andesite,  dacite,  and  rhyolite. 

It  is  usually  perfectly  fresh,  without  signs  of  decomposition  ; 
occasionally  a  small  zone  of  the  surrounding  rock  is  stained  ochre- 
yellow.  In  the  granite  from  Ilchester,  Md.,  allanite  with  pronounced 
zonal  structure  occurs  at  the  centre,  of  epidote  crystals,  the  two 
minerals  having  parallel  crystallographic  orientation.* 

Allanite  may  be  confused  with  biotite  and  hornblende  in  certain 
instances  when  they  possess  the  same  reddish  brown  color  and  do  not 
exhibit  their  characteristic  cleavage  or  crystal  form,  but  it  may  be 
distinguished  from  basal  sections  of  biotite  by  its  strong  pleochroism 
and  larger  optic  axial  angle,  and  from  hornblende  by  its  higher  double 
refraction. 


*  Wm.  H.  Hobbs,   On  the  rocks  occurring  in  the  neighborhood  of  Ilchester, 
Howard  Co.,  Md. ;  preliminary  notice.  The  Johns  Hopkins  University  Circulars,  No. 
65,  Apr.  1888. 
18 


274        PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

Titanite. 

Titanite  is  only  an  accessory  constituent  of  those  rocks  in  which  it 
occurs,  but  it  is  at  times  a  very  abundant  one.  When  it  occurs  in 
eruptive  rocks  as  a  primary  component,  it  is  always  well  crystallized, 
and  belongs  to  the  oldest  secretions  from  the  magma.  Less  frequently 
it  forms  regular  crystals  in  the  Archaean  rocks.  In  both  groups  of 
rocks  it  is  common  in  the  form  of  irregular  grains  as  a  secondary 
product  from  titaniferous  magnetite,  from  ilmenite  and  rutile.  The 
regular  boundaries  of  the  primary  crystals  also  are  occasionally  more 
or  less  destroyed  through  mechanical  and  chemical  processes.  The 
forms  of  embedded  titanite  crystals  are  less  variable  than  those  of 
attached  ones,  but  there  is  a  certain  variableness  even  in  these.  The 
most  predominant  type  is  represented  in  Fig.  95  ;  besides  n  =  f  P2  (123) 
there  appear  less  prominently  P  =  oP  (001)  and  y  —  Poo  (101),  less 
frequently  x  =  -JP55  (102)  and  r  =  Poo  (Oil).  rJ^ie  combination 
I  —  GO  P  (110)  twith  n  (Fig.  96),  besides  other  subordinate  faces,  is 


especially  met  with  in  amphibolites  and  mica  schists.  The  combination 
y,  7i,  r,  also,  is  not  uncommon.  The  angles  most  important  for  cross- 
sections  are  lf\l  =  133°  52',  nf\ri=  136°  12',  P /\y  =  60°  17',  P /\x  = 
39°  17'.  The  commonest  sections  are  acute  rhombs,  and  long,  lath-shaped 
ones  with  pointed  ends.  Twinning  is  not  infrequent,  but  is  never 
recognized  by  the  outline,  only  by  the  behavior  in  polarized  light. 
The  twinning  boundary  always  bisects  the  acute  angle  of  the  rhombs 
(PI.  XXIII.  Fig.  1).  Hence  the  base  appears  to  be  the  twinning 
plane.  Zonal  structure  is  seldom  observed ;  kernel  and  shell  are  then 
separated  from  each  other  by  the  faces  n  or  Z,  and  spring  apart  upon 
being  struck. 

Titanite  only  appears  in  the  form  of  granular  or  short  columnar 
aggregates  when  it  forms  pseudomorphs  after  one  of  the  above-named 
minerals. 

The  cleavage  along  the  prism  I  only  shows  itself  by  occasional 
rough  cracks ;  since  the  prism  seldom  occurs  as  a  predominant  form, 


TITANITE.  275 

the  cleavage  is  not  parallel  to  the  boundary,  which  is  usually  deter- 
mined by  n, — a  phenomenon  characteristic  of  titanite  (PI.  XI.  Fig.  3). 
Cleavage  is  rarely  observed  on  secondary  grains  and  aggregates  of 
titanite. 

The  titanite  of  rocks  becomes  transparent  and  colorless  to  white, 
yellowish,  or  reddish ;  its  transparency,  however,  is  generally  small. 
The  index  of  refraction  is  very  high,  ft  =  1.905-1.910 ;  the  marginal 
total  reflection  and  the  rough  character  of  the  surface  in  Canada 
balsam  are  greater  than  for  epidote.  The  double  refraction  has 
not  yet  been  measured,  but  does  not  appear  to  be  great ;  the  interfer- 
ence colors  are  only  striking  in  sections  parallel  to  the  axial  plane, 
otherwise  they  are  but  slightly  noticeable  on  account  of  the  strong 
dispersion  p  >  v .  The  optic  axes  lie  in  the  clinopinacoid ;  thus 
1)  —  fc,  and  the  positive  acute  bisectrix  is  normal  to  x  =  ^Poo  (102), 
from  which  is  derived  the  scheme  Fig.  97.  The  dispersion  of  the 
optic  axes  for  different  kinds  of  light  is  greater  than  for  any  rock- 
making  mineral,  and  furnishes  a  positive 
means  of  determination.  Des  Cloizeaux 
measured  %EP  =  53°,  and  on  another 
crystal  55° -56°,  and  ZEV  =  32°  27'  and 
34°.  The  dispersion  of  the  bisectrices  is 
scarcely  noticeable.  In  convergent  light  Klsf*  97 

all   the  acutely   rhombic   sections   from 

the  orthodiag'onal  zone  give  axial  bars,  axial  figures  or  loci  of  bisectri- 
ces, from  which  it  can  be  seen  that  the  axial  plane  bisects  the  obtuse 
angle, — a  convenient  means  of  distinction  from  staurolite,  which 
is  otherwise  quite  similar.  Sections  lying  approximately  in  the  face  so 
show  an  interference  figure,  whose  hyperbolas  are  not  black  in  the 
diagonal  position  because  of  the  strong  dispersion,  but  are  red,  green, 
and  blue  from  the  inside  outward.  By  using  red  and  blue  glasses  the 
great  difference  in  the  axial  angle  for  the  two  colors  can  readily  be 
seen. 

The  extinction  angles  of  the  different  sections  are  not  characteristic. 
When  the  section  is  considerably  inclined  to  the  axial  plane,  there  is 
no  complete  extinction  in  white  light,  because  of  the  strong  axial 
dispersion. 

The  pleochroism  is  scarcely  noticeable  in  very  thin  sections  and  for 
pale  coloring :  the  strongly  colored  crystals  have  C  =  red,  with  a  tinge 
of  yellow ;  fc  =  yellow,  often  with  a  tinge  of  greenish ;  a  —  almost 
•colorless. 

H.  =  5-5.5.     Sp.  gr.  =  3.4-3.6.      Chemical  composition  =  CaO, 


276        PHJSIOaRAPHY  OF  THE  HOCK-MAKING  MINERALS. 

SiO2,  TiO2.  Not  attacked  by  hydrochloric  acid.  Decomposed  by 
sulphuric  acid ;  the  solution  becomes  orange  yellow  upon  the  addition 
of  hydrogen  superoxide.  On  account  of  its  density  it  falls  with  the 
ferruginous  constituents  in  the  heavy  solutions,  and  can  generally  be 
easily  separated  from  these  by  the  electro-magnet. 

Upon  decomposition  titanite  bleaches  and  loses  its  lustre ;  at  the 
same  time  carbonate  of  lime  separates  out.  The  dull  secondary  sub- 
stance has  not  been  investigated.  In  other  instances  of  decomposi- 
tion an  opaque  iron-ore,  probably  ilmenite,  is  deposited  on  the  cleavage 
cracks.  Its  alteration  into  rutile  has  been  observed  by  P.  Mann*  in 
elaeolite  syenites;  a  decomposition  of  titanite  with  the  production 
of  anatase  was  observed  by  J.  S.  Dillerf  in  the  amphibole  granitites 
of  the  Troad,  Greece. 

The  titanite  of  eruptive  rocks  encloses  the  older  constituents 
associated  with  it,  as  the  ores,  apatite  and  zircon,  rarely  glass  and  fluid 
inclusions ;  in  the  Archaean  rocks  it  is  generally  free  from  inclusions. 

Its  distribution  is  considerable :  it  occurs  in  the  acid  rocks  which 
are  not  too  poor  in  magnesia  and  iron,  as  granitites,  amphibole  granites, 
syenites,  diorites,  trachytes,  and  abundantly  in  the  elseolite  syenites  and 
phonolites ;  it  is  rarer  in  the  corresponding  porphyritic  rocks.  It  is 
absent  from  the  basic  eruptive  rocks  rich  in  ilmenite  and  titaniferous 
magnetite.  .Among  the  Archaean  rocks  also  it  -occurs  to  a  notable 
extent  in  rocks  rich  in  MgO  and  FeO,  that  is,  in  the  biotite  and  am- 
phibole-bearing  gneisses  and  schists.  As  a  secondary  product  it  is 
found  in  all  rocks  bearing  ilmenite  and  rutile. 

Monodinic   JFeldspars. 
Literature. 

A.  DBS  CLOIZEAUX,  Observations  sur  les  modifications  permanentes  et  temporaires 
que  Faction  de  la  chaleur  apporte  a  quelques  proprietes  optiques  de  plusieurs 
corps  cristallises.  Ann.  des  Mines.  1862.  II. 

—  Nouvelles  recherches  sur  les  proprietes  optiques  des  cristaux  naturels  ou  arti- 

ficiels,  et  sur  les  variations  que  ces  proprietes  eprouvent  sous  1 'influence  de  la 
chaleur.     Mem.  Sav.  etrangers.  Paris.   1867.  XVIII. 

—  Examen  microscopique  de  I'orthose  et  des  divers  feldspaths  tricliniques.     C.  R. 

1876.  LXXXII.  1017-1022. 
A.  MICHEL-LEVY,  De  1'emploi  du  microscope  polarisant  a  lumiere  parallele  pour  la 

determination  des  esp^ces  minerales  en  plaques  minces.     Ann.  des  mines.  1877 

(7.)  XII.  392-471. 

G.  TSCHERMAK,  Die  Feldspathgruppe.     S.  W.  A.  1864.  December.  L. 
CH.  E.  WEISS,  Beitrage  zur  Kenntniss  der  Fcldspathbildung.     Haarlem.  1866. 

*  N.  J.  B.  1882.  II.  290. 
t  N.  J.  B.  1883.  I.  187. 


MONOCLINIC  FELDSPARS. 


277 


The  monoclinic  feldspars  are  classed  as  orthoclase  or  sanidine, 
according  to  whether  they  occur  in  the  older  massive  rocks  and 
Archaean  rocks  or  in  younger  volcanic  rocks.  With  this  difference  in 
geological  position  are  connected  certain  peculiarities  in  habit  and  in 
physical  behavior.  For  simplicity  of  expression,  the  term  orthoclase 
will  be  here  used  for  all  monoclinic  potash  feldspars,  including  sani- 
dine, while  the  name  sanidine  will  be  confined  to  the  latter  variety  of 
feldspar. 

Orthoclase  always  appears  in  rocks  with  more  or  less  complete 
crystallographic  boundaries,  whenever  they  possess  a  distinctly  porphv- 
ritic  structure  ;  the  outward  form  disappears  more  and  more  as  the 
structure  becomes  more  distinctly  granular.  In  the  schistose  rocks  of 
the  Archaean  the  orthoclase  is  generally  not  crystallographically 
bounded.  But  a  distinct  crystal  form  is  also  developed  here  when- 
ever a  porphyritic  structure  occurs. 

The  crystals  of  embedded  orthoclase  always  show  the  faces  P  =  oP 
(001),  M=  ooP^o  (010),  predominant;  1=  ooP(llO),  x  =  P£  (101), 
y  —  2Poo  (201),  more  subordinate ;  rarely  n  =  2Poo  (021),  o  =  P  (111), 
and  in  the  zone  I :  M,  z  =  ccPS  (130).  The  angles  important  for 
cross-sections  are  l/\l  =  118°  48',  l/\M  =  119°  36',  P J\x  =  129°  40', 
P  A  y  =  99°  37',  P  A  M  =  90°.  The  faces  P  and  x  are  almost  equally 
inclined  to  the  vertical  axis.  The  habit  of  the  crystals  is  either  more 


M 


.  98 


Fig.  99 


or  less  tabular  parallel  to  M  (Fig.  98),  or  prismatic  parallel  to  the  axis 
a  (Fig.  99).  The  shape  of  sections  in  different  directions  is  evident 
from  the  figures. 

The  commonest  variety  of  twinning  is  that  according  to  the  Carls- 
bad  law.  The  twinning  axis  is  the  vertical  axis,  and  the  twinned  in- 
dividuals either  join  along  the  plane  of  symmetry  or  penetrate  each 
other  irregularly.  The  characteristics  of  these  twins  is  that  the  basal 
faces  slope  in  different  directions.  In  the  orthoclases  of  many  rocks 


278        PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

(granite  of  Elba)  the  faces  P  and  x  lie  apparently  parallel  (Fig.  100). 
In  cross-sections  the  twinning  boundary  either  lies  parallel  to  the  in- 
tersection of  M  and  the  cutting  plane,  or  it  is  an  irregularly  bent  or 
jagged  line.  The  twin  character  is  often  not  recognizable  in  the  out- 
line of  these  sections  when  the  crystal  is  a  contact  twin  ;  but  it  i& 
shown  by  the  cleavage  and  optical  behavior  (PL  XXIII.  Fig.  2). 

The  Baveno  law,  by  which  the  normal  to  n  is  the  twinning  axis,  is 
far  more  rare  in  rocks,  and  is  always  sporadic.  The  twinning  plane  n 
is  also  the  composition  plane  (Fig.  101),  and  since  the  faces  inclined 
to  the  axis  d  are  seldom  well  developed,  and  n  /\n  is  almost  90°,  it 
happens  that  the  outline  of  the  sections  give  no  indication  of  the 
twinning.  The  basal  faces  stand  at  right  angles  to  one  another,  and 
since  M  is  also  a  cleavage  face  the  twinning  cannot  be  detected  by  the 
cleavage,  but  is  found  through  the  optical  behavior.  Twins  of  this 


Fig.  101 

kind  are  always  prismatic  in  the  direction  of  d.  Hence  the  cross- 
sections  are  mostly  square,  or  rhombic,  with  the  trace  of  the  twin- 
ning plane  running  diagonally  through  them  (PL  XXIII.  Fig.  3). 
Through  the  repetition  of  this  law  along  one  or  two  more  faces  of  ny 
there  arise  trillings  and  f curlings  which  are  only  recognized  optically. 

The  rarest  twinning  is  according  to  the  Manebacher  law  ;  twinning 
axis  is  the  normal  to  P.  P  is  also  composition  plane,  and  the  twin- 
ning is  rarely  recognized  by  the  outline.  The  cleavage  is  the  same  in 
both  parts  of  the  twin,  and  the  optical  behavior  alone  reveals  the 
twinning.  The  trace  of  the  twinning  plane  is  parallel  to  that  of  P  in 
the  section.  This  law  only  occurs  in  a  few  rocks,  but  then  quite  fre- 
quently. The  quartz  porphyries  are  the  principal  rocks  which  show  it. 

The  combination  of  Carlsbad  and  Baveno  laws  is  not  infrequent. 
Confused  in tergrowths,  which  are  probably  brought  about  by  twinning, 
have  been  observed  quite  frequently,  but  can  seldom  be  made  out  from 


MONOCL1NIC  FELDSPARS.  279 

the  cross-sections.  Such  exceptional  twinnings  have  been  described 
by  Klockmann*  arid  Haushofer.f 

The  dimensions  of  the  crystals  vary  extraordinarily,  and  the  crystalli- 
zations of  the  second  and  third  generation  in  porphyritic  eruptive  rocks 
are  of  ten  extremely  small,  even  for  microscopical  examination.  Neverthe- 
less, true  incipient  forms  of  growth  or  skeleton  crystals  of  orthoclase 
have  not  yet  been  observed  with  certainty.  The  habit  is  always  that 
of  the  larger  crystals.  Still,  in  the  older  porphyries  the  tabular  forms 
predominate,  while  in  the  trachytes  and  phonolites  it  is  prismatic  ;  in 
the  rhyolites  both  occur,  but  almost  never  together.  The  minute 
prisms  often  group  themselves  together  in  radially  columnar  aggregates. 
They  form  spherulites,  which  either  lie  free  in  the  rock  or  attach 
themselves  in  tufts  to  the  older  orthoclase  crystals.  Such  orthoclase 
microlites,  like  all  nearly  trichitic  forms,  often  exhibit  a  fraying  out 
into  diverging  curved  processes. 

Zonal  structure  is  very  common,  indicating  the  original  crystal 
form  when  this  has  been  destroyed  by  subsequent  changes.  If  the 
crystals  are  perfectly  fresh  it  is  often  unnoticeable ;  it  then  first 
appears  when  the  condenser  is  lowered  in  order  to  produce  strongly 
divergent  rays,  or  when  the  crystal  is  observed  between  crossed  nicols 
in  a  semi-dark  position.  It  becomes  very  distinctly  marked  through 
interpositions,  especially  glass  inclusions  in  sanidines,  and  by  the  first 
stages  of  decomposition  (Fl.  XXIII.  Fig.  4). 

The  crystal  form  of  orthoclase  is  often  completely  rounded  through 
chemical  corrosion  by  the  magma,  brought  about  by  changes  in  its 
composition  or  physical  condition  ;  in  this  way  occasionally  they  be- 
come spherical  grains  (as  in  many  quartz  porphyries).  Mechanical 
deformations  are  very  common  in  the  porphyritic  eruptive  rocks ;  the 
crystals  are  broken  in  consequence  of  movements  of  the  magma 
enclosing  them,  and  become  angular  and  sharp-edged  grains  (PL 
XXIII.  Fig.  5).  In  the  Archaean  rocks  the  mechanical  processes  which 
have  been  active  in  mountain-making  have  often  produced  a  marginal 
rubbing  and  crushing  of  the  orthoclase  crystals  (PL  IY.  Fig.  2), 
which  may  lead  to  their  complete  destruction,  so 'that  an  originally 
simple  individual  is  converted  into  a  confused  aggregate.  In  other  in- 
stances these  processes  only  lead  to  small  molecular  displacements  and 
strains,  which  are  first  recognized  by  optical  phenomena,  such  as  the 

*  Die  Zwillingsverwachsungen  des  Orthoklases   auz  dem  Granitit  des  Riesen- 
gebirges.     Z.  X.  1882.  VI.  493-510. 

f  Orthoklaszwillinge  von  Fichtelberg.     Z.  X.  1879.  III.  601. 


280        PHYSIOGRAPHY  OF  THE  ROCK-MAKING   MINERALS. 

gradually  changing  orientation  of  the  axes  of  elasticity  and  the  result- 
ing undulatory  extinction  (PI.  IV.  Fig.  2). 

The  cleavage  of  the  feldspars  parallel  to  the  faces  P  and  M  is  one 
of  the  most  important  factors  in  their  microscopical  diagnosis.  The 
cleavage  parallel  to  P  is  the  most  perfect  and  easiest ;  that  parallel  to 
M  varies  somewhat,  and  occasionally  readies  the  perfection  of  that 
parallel  to  P.  But  both  cleavages  are  not  complete  enough  to  become 
noticeable  in  thick  sections.  When  sufficiently  thin,  both  cleavages 
show  themselves  in  quite  sharp  and  straight  cracks  (PL  XL  Fig.  1), 
which  are  somewhat  more  numerous  and  continuous  parallel  to  P  than 
parallel  to  M.  In  many  sanadines  and  orthoclases,  however,  P  and  M 
can  scarcely  be  distinguished  from  one  another  by  their  cleavages.  The 
cleavage  cracks  lie  parallel  to  one  another  in  all  sections  from  the  zone 
P  and  M ;  they  intersect  at  right  angles  in  all  sections  from  the  zone 
oP :  ooPoo  (001 : 100).  In  sections  from  the  zone  ooPoo  :  ooPob 
(100 : 010)  they  intersect  at  angles  which  vary  from  90°  to  63°  53' 
(ft  =  63°  53').  * 

In  the  sanidines  there  is  frequently  a  rude  parting  approximately 
parallel  to  oo  Pa5  (100).  The  corresponding  cracks  are  never  straight 
nor  strictly  parallel  (PI.  XXIII .  Fig.  6) ;  they,  however,  appear  con- 
siderably sooner  in  thin  sections  than  the  cleavage  cracks  parallel  to  P 
and  J^,  and  are  the  only  ones  noticeable  in  the  thicker  sections. 

The  orthoclases  become  transparent  and  colorless.  Their  index  of 
refraction  is  small,  very  rarely  the  same  as  that  of  Canada  balsam  ; 
their  double  refraction  is  weak  to  very  weak — weaker  than  that  of 
quartz  and  the  lime-soda  feldspars.  Des  Cloizeaux  determined  : 

On  adular  from  St.  Gotthard ana  =  1.5190    /3na  =  1.5237    yna  =  1. 5260 

On  sanidine  from  Wehr,  with  normal 
symmetrical  axial  position ap  =  1.5170  ftp  =  1.5239  y?  =  1.5240 

The  same,  with  symmetrical  axial  posi- 
tion   av  =1.5256  flv  =1.5355  yv  =1.5356 

y— a  under  normal  conditions  varies  between  0.007-0.005.  and  the 
interference  colors  do  not  exceed  the  1st  order  even  in  thick  sections ; 
in  good  sections  they  reach  yellow  of  the  1st  order  at  most. 

All  orthotomic  feldspars  are  optically  negative ;  in  general  the 
plane  of  the  optic  axes  is  normal  to  the  plane  of  symmetry,  and  forms 
an  angle  of  3°-7°,  with  the  plane  of  the  cry  stall  ographic  axes  d  and  b  in 
the  obtuse  angle  /?.  In  exceptional  instances,  apparently  when  the 
percentage  of  soda  is  high,  this  inclination  increases  to  10°-12°.  The 
horizontal  dispersion  is  very  noticeable,  and  p  >  v .  The  axial  angle 
varies  within  wide  limits;  it  is  always  large  for  orthoclases  proper, 


MONOCL1NIC  FELDSPARS. 


281 


%£"=  119°-125°  ;  and  for  sanidines  it  varies  in  crystal  from  the  same 
rock,  even  in  plates  of  the  same  crystal,  but  is  always  smaller,  between 
50°  and  0°  in  air.  The  scheme  Fig.  102  illustrates  these  relations  ;  it 
is  evident  that  the  obtuse  positive  bisectrix  emerges  from  the  face  J/, 
while  plates  parallel  to  the  parting  face,  which  is  approximately 
parallel  to  the  orthopinacoid,  lie  nearly  at  right  angles  to  the  acute 
negative  bisectrix,  and  give  an  interference  figure.  In  the  same  way 
all  sections  from  the  prism  zone  show  axial  bars  or  the  loci  of  axes 
slightly  eccentric  to  the  field  of  view.  The  trace  of  the  axial  plane 
lies  nearly  parallel  to  the  most  perfect  cleavage. 

In  all  sections  from  the  orthodiagonal  zone  the  extinction  is  parallel 
and  normal  to  the  perfect  cleavage ;  in  the  zone  P  :  M  it  makes  small 
angles  with  the  parallel  cleavages  along  P  and  M  which  increase  from  0° 
on  P  to  3°-7°,  seldom  to  12°  on  M.  In  the  zone  ooPci  :  ooP^o  it 
is  better  to  measure  the  extinction  angle  from  the  second  cleavage 


.  103 


.  1O3 


along  M ;  when  near  this  face  it  is  21°,  increases  slowly  until  a  section 
is  reached,  which  is  inclined  45°  to  J/,  then  rapidly  to  90°. 

In  Carlsbad  twins  the  traces  of  the  axial  planes  of  the  twinned  indi- 
viduals are  parallel.  In  parallel  light  both  individuals  extinguish  simul- 
taneously in  the  zone  oP :  ooPoo  (001  : 100),  and  the  cleavage  cracks 
lie  parallel  to  the  directions  of  extinction.  In  the  zone  ooP<sb  :  ooPoo 
(100  :  010)  the  cleavage  cracks  and  extinctions  lie  parallel  and  normal 
to  the  twinning  line  on  the  first-named  face ;  on  the  second  face  the 
cleavage  along  M  is  parallel  to  the  twinning  line  ;  that  along  P  makes 
an  angle  of  127°  46'  =  2<)^  A  and  is  bisected  by  the  twinning  line  (Fig. 
103).  The  directions  of  extinction  of  the  two  individuals  make  an 
angle  of  2  X  21°  =  42°,  which  is  also  bisected  by  the  twinning  line. 
In  sections  from  this  zone  with  varying  inclination  to  M  the  angle 
between  the  cleavages  increases  from  127°  46'  to  180°;  the  angle  between 
the  directions  of  extinction  increases  from  42°  to  180°,  at  first  slowly, 
then  very  rapidly.  The  twinning  line  always  bisects  the  cleavages  and 
extinction  angles  (PL  XXIII.  Fig.  2).  The  zone  P  :  Mis  not  common 


282         PHYSIOGRAPHY  OF  THE  BOCK-MAKING  MINERALS. 


to  both  individuals  of  a  Carlsbad  twin  ;  the  zone  P :  M  of  one  individual 
is  approximately  the  zone  x :  m  of  the  other.  In  the  first  individual 
all  sections  have  parallel  cleavage  cracks,  with  which  the  directions  of 
extinction  make  angles  of  0°  on  P  to  3°-7°  or  rarely  12°  on  M ;  in  the 
second  individual,  since  a.  basal  section  of  the  first  one  is  approximately 
parallel  to  x  of  the  second,  it  will  show  rectangular  cleavages  and  the 
extinction  parallel  to  them.  "With  increasing  inclination  to  M  the 
cleavage  angle  decreases  to  54°.  The  extinction  angle  measured  from 
the  twinning  line  increases  from  0°  to  74°  on  the  same  side  as  in  the 
first  individual. 

In  Baveno  twins  only  the  cross-sections  and  the  zone  P  :  M  are  of 
consequence.  In  cross-sections  the  twinning  line  is  diagonal  to  the 
cleavage  (PL  XXIII.  Fig.  3)  and  to  the  outline,  and  in  convergent 
light  there  are  two  interference  figures  standing  at  right  angles  to  one 


another  (Fig.  104).  In  sections  from  the  zone  P  :  M  the  cleavages  of 
both  individuals  are  parallel  to  one  another  and  to  the  twinning  line; 
the  extinction  angle  in  one  individual  increases  from  0°  to  3°-7°,  while 
in  the  other  it  decreases  within  the  same  limits.  The  maximum  ex- 
tinction angle  in  one  individual  coincides  with  the  minimum  in  the 
other. 

The  optical  behavior  of  the  Manebacher  twins  is  easily  under- 
stood from  what  has  been  said.  They  have  the  zones  P :  M  and 
oP  :  c»_Po6  in  common.  The  zone  M  :  <x>Poo  of  one  corresponds  to 
a  zone  M  :  mP&>  of  the  other,  which  is  not  characteristic. 

Occasionally  in  the  sanidines  of  lavas,  more  frequently  in  those 
sanidines  which  have  been  thrown  out  loose,  and  in  those  of  lapilli,  the 
axial  plane  lies  in  the  plane  of  symmetry  (Fig.  105),  that  is,  normal  to 
the  most  perfect  cleavage.  The  dispersion  is  then  p  <  v.  The  axial 
angle  is  always  small,  2^—  40°-0° ;  indeed,  instances  occur  in  which 
the  axial  plane  for  red  light  is  normal  to  M,  while  that  for  blue  is 
parallel  to  M.  The  orientation  of  the  axes  of  elasticity  is  nearly  the 
same  in  all  cases.  The  inclined  dispersion  which  accompanies  the 
symmetrical  axial  position  is  not  usually  great. 


MONOCLINIC  FELDSPARS.  283 

The  orthoclases  exhibit  no  pleochroism  nor  noticeable  difference  of 
absorption. 

Des  Cloizeaux  has  shown  that  by  raising  the  temperature  the  axial 
angle  of  feldspars  diminishes  so  long  as  the  position  of  the  axial  plane 
is  normal-symmetrical,  and  increases  when  it  coincides  with  the  plane 
of  symmetry.  With  sufficient  heating  the  axial  angle  of  normal-sym- 
metrical axes  decreases  gradually  to  0°  for  all  colors  commencing  with 
blue;  the  axes  pass  into  the  plane  of  symmetry  without  noticeably 
changing  the  position  of  the  obtuse  bisectrix,  and  gradually  open  as- 
the  temperature  increases.  Upon  cooling,  the  axes  return  to  their 
original  position  if  the  temperature  has  not  exceeded  500°  C.  If  the 
temperature  is  kept  at  from  600°-1000°  C.  for  some  time,  the  resulting 
changes  remain  fixed,  and  do  not  alter  upon  cooling. 

The  specific  gravity  of  sanidine  and  orthoclase,  when  unaltered,  is- 
the  same,  2.54—2.56.  This  permits  a  mechanical  separation  from  the 
lime-soda  feldspars  without  difficulty.  Chemical  composition,  K2O, 
A12O3,  6SiO2 ;  but  this  is  always  isomorphously  mixed  with  a  variable 
amount  of  a  similarly  constituted  soda  molecule.  Since  mechanical 
intergrowths  with  a  soda  feldspar  are  also  quite  common,  it  cannot  be 
seen  from  the  analyses  to  what  extent  soda  has  replaced  potash.  Or- 
thoclase is  riot  noticeably  attacked  by  hydrochloric  acid  even  when 
heated,  but  it  is  very  readily  decomposed  by  hydrofluoric  acid. 

Sanidine  occurs  in  rocks  either  as  older  secretions  or  as  a  later 
crystallization  of  the  groundmass.  In  the  first  case  it  has  exactly  the 
form  of  the  macroscopic  crystals,  quite  thinly  tabular  parallel  to  M  OY 
slender  prismatic  parallel  to  d.  Its  crystallization  has  followed  that 
of  the  ferruginous  constituents,  of  the  haiiyne  minerals,  of  nepheline, 
and  to  some  extent  that  of  the  plagioclases ;  it  preceded  that  of  quartz. 
These  secretions  are  occasionally  free  from  inclusions,  and  then  they 
are  pellucid.  More  frequently  they  enclose  the  associated  minerals,  and 
especially  gas  and  glass  inclusions,  the  latter  often  more  or  less  devit- 
rified.  The  shape  of  these  inclusions  is  either  irregular  or  is  borrowed 
from  their  host,  and  then  shows  the  combination  P,  J/",  y,  I.  Fluid  in- 
clusions are  rare.  The  arrangement  of  these  inclusions  is  seldom  ir- 
regular; they  generally  lie  in  concentric  zones,  or  are  crowded  together 
centrally  or  peripherally.  Occasionally  (Drachenfels)  they  are  dis- 
tributed in  layers  parallel  to  M,  less  frequently  parallel  to  P. 

Regular  intergrowths  of  sanidine  crystals  with  triclinic  feldspars 
are  very  common,  and  though  apparently  very  diversified,  always  follow 
the  law  that  both  feldspars  have  M  and  the  edge  M :  I  in  common. 
This  intergrowth  may  amount  to  a  complete  envelopment  (PI.  XXIY. 


284         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

Fig.  1),  in  which  the  sanidine  is  almost  always  on  the  outside,  ver) 
seldom  on  the  inside  ;  or  the  feldspars  may  join  one  another  only  along 
one  side,  or  they  may  penetrate  each  other  with  irregular  boundaries, 
so  that  in  thin  section  they  mutually  enclose  one  another  in  irregularly 
shaped  patches.  Sanidine  very  rarely  exhibits  a  microline-like  struc- 
ture such  as  Mugge*  described  in  the  olivine-bearing  trachytes  from 
Fayal. 

When  sanidine  occurs  in  a  second  generation  in  the  groundmass  it 
is  usually  free  from  inclusions. 

In  general,  the  sanidines  exhibit  no  signs  of  decomposition  ;  an  al- 
teration into  zeolitic  aggregates  is  quite  common  in  phonolites  (PL 
XXIY.  Fig.  2).  The  red  color  occurring  in  some  sanidines  arises  from 
infiltrations  of  iron  oxide. 

Orthoclase,  even  when  perfectly  fresh,  does  not  have  the  glassy  habit 
of  sanidine,  or  the  parting  along  a  face  approximately  parallel  to  the 
orthopinacoid.  The  perfectly  fresh  examples  resemble  adular.  It  is 
convenient  to  separate  the  orthoclase  of  porphyritic  rocks  from  that 
of  granular  rocks ;  with  the  latter  is  closely  related  the  orthoclase  of 
Archaean  rocks. 

The  orthoclase  of  porphyritic  rocks  resembles  sanidine  in  its  forms, 
when  it  occurs  as  porphyritic  crystals.  But  inclusions  are  much  less 
abundant,  and  glass  inclusions  can  seldom  be  recognized  as  such  on 
.account  of  the  state  of  preservation  of  the  rocks.  The  orthoclase  of 
later  generation  is  free  from  inclusions,  and  is  more  equally  developed 
in  all  directions  than  the  sanidine.  The  intergrowths  with  triclinic 
feldspars  are  analogous  to  those  of  sanidine;  mutual  penetrations 
with  quartz  are  very  frequent,  and  are  known  as  granophyric  inter- 
growths  (PI.  VIII.  Fig.  3).  Intergrowths  with  microcline  only  occur 
in  those  porphyritic  rocks  which,  like  granite  porphyry,  are  very  closely 
related  to  granular  rocks. 

The  orthoclase  of  granular  rocks  and  of  Archaean  rocks  shows  but 
imperfect  crystallographic  boundaries  or  none  at  all ;  glass  inclusions 
never  occur.  On  the  other  hand,  fluid  inclusions  are  very  common  in 
fresh  orthoclases,  but  disappear  in  the  processes  of  alteration.  Besides 
the  older  associated  minerals,  orthoclase  occasionally  encloses  scales  of 
specular  iron  and  microlitic  interpositions.  But  this  is  always  a  local 
or  individual  phenomenon,  not  a  general  one.  The  arrangement  of  the 
inclusions  in  orthoclase  also  is  generally  regular,  zonal,  central,  or  pe- 
ripheral. The  tendency  of  orthoclase  to  form  an  intergrowth  with 

*  N.  J.  B.  1883.     II.  204. 


MONOCLINIC  FELDSPARS.  285 

triclinic  feldspar  is  quite  extraordinary.  As  with  sanidine,  it  is  either 
an  envelopment  of  one  by  another — the  rarest  case — or  a  simple  jux- 
taposition; or  finally  a  complete  penetration,  the  last  being  the  com- 
monest case.  The  combined  feldspars  always  have  the  second  cleavage 
face  M  and  the  edge  M:  I  in  common.  These  intergrowths  are  gen- 
erally only  perceptible  in  polarized  light  because  of  the  great  similarity 
in  the  form  of  all  the  feldspars ;  in  many  cases,  however,  they  can  be 
recognized  microscopically  by  dull  places  on  the  principal  cleavage 
face,  or  by  a  banded  appearance  on  the  second  cleavage  face. 

Microcline,  albite,  and  oligoclase  are  known  to  take  part  in  such  inter- 
growths  with  orthoclase.  PI.  XX I Y.  Fig.  3  gives  an  example  of  the 
penetration  of  orthoclase  and  plagioclase.  In  sections  parallel  to  P  the 
orthoclase  is  recognized  by  its  extinction  parallel  to  the  cleavage  along  M, 
while  the  plagioclases  and  microcline  extinguish  more  or  less  obliquely 
to  this  cleavage.  The  cleavage  along  M  passes  uninterruptedly  through 
the  different  feldspars.  In  sections  parallel  to  M  the  cleavage  along  P 
runs  only  approximately  parallel  through  orthoclase  and  microcline  on 
one  side  and  albite  and  oligoclase  on  the  other.  In  such  sections  ortho- 
clase and  microcline  are  distinguished  from  one  another  with  difficulty, 
while  albite  and  oligoclase  are  easily  determined  by  their  different  ex- 
tinction angles.  In  chance  sections/  the  intergrowth  is  recognized  by 
the  different  extinction  angles  in  the  different  feldspars,  in  part  also 
by  the  local  abundance  of  the  twin  lamellae  of  plagioclase,  and  by  the 
differences  in  the  interference  colors.  But  the  determination  of  the  com- 
ponent individuals  can  seldom  be  made  with  certainty  in  such  sections. 

The  lamellar  intergrowth  of  orthoclase  (with  or  without  micro- 
cline) and  albite,  like  that  which  exists  macroscopically  in  perthite,  is 
particularly  common.  The  albite  lamellae  are  often  so  extremely  fine 
that  they  are  not  perceptible  as  such  with  low  magnifying  powers. 
They  appear  to  lie  parallel  to  the  prism  or  orthopinacoidal  faces  in  or 
thoclase,  which  then  assumes  a  striated  appearance  in  sections  from  the 
prism  zone  (PL  XXIY.  Fig.  4).  When  the  lamellae  are  still  smaller 
these  sections  appear  finely  fibrous,  and  exhibit  very  different  degrees 
of  brightness,  according  to  whether  the  light  travels  parallel  or  perpen- 
dicular to  the  longer  direction  of  the  lamellae — as,  for  example,  in  the 
feldspars  of  many  Saxon  granulites.  Finally,  these  albite  lamellae 
reach  such  minuteness  that  they  are  only  recognized  as  such  by  very 
high  magnifying  powers ;  then  the  orthoclase  occasionally  exhibits  a 
beautiful  blue  lustre  on  the  orthopinacoid  and  on  faces  lying  near  it,  as 
in  many  adulars,  the  moonstone  of  Ceylon,  and  the  schillerizing  ortho- 
clases  of  Frederiksvarn.  It  is  possible  that  these  submicroscopic 


286         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

albite  lamellae  explain  the  high  extinction  angle  on  M  in  such  feld- 
spars, which  is  nearly  the  mean  of  the  values  for  orthoclase  and  albite. 
This  microscopic  lamellar  intergrowth  is  called  microperthite.  PI. 
XXIV.  Fig.  5  shows  such  microscopic  mixtures  of  orthoclase  and  albite 
in  sections  in  different  directions.  In  approximately  basal  sections  it 
is  seen  that  the  albite  forms  thin  rods ;  when  of  larger  dimensions  they 
become  small  lamellae  and  spindle-shaped  bodies.  An  acid  lime-soda 
feldspar  also  forms  microperthitic  intergrowths. 

The  dull  and  cloudy  appearance  of  orthoclase  is  due  to  a  more  or 
less  advanced  alteration  into  muscovite  or  kaolin.  The  two  processes, 
which  are  so  closely  related  chemically,  and  arise  from  a  partial  or  total 
removal  of  the  potash  by  water,  together  with  the  separation  of  4SiO2, 
exhibit  the  greatest  similarity  morphologically,  and  are  scarcely  deter- 
minable  microscopically.  In  both  cases  there  form  along  the  cleavage 
cracks  aggregates  of  a  perfectly  uniform  substance,  which  is  colorless 
and  is  strongly  doubly  refracting.  The  feldspar  appears  to  be  dis- 
tended, and  is  the  more  opaque  and  earthy  the  liner  the  scaly  structure 
of  the  secondary  product.  The  process  often  commences  in  the  centre 
of  the  orthoclase  crystal,  especially  when  there  were  many  central  in- 
clusions, so  that  the  attackable  surface  was  as  great  as  possible. 

In  the  alteration  to  kaolin  the  dimensions  of  the  secondary  prod- 
ucts are  always  smaller  than  in  that  to  muscovite.  They  can  be  dis- 
tinguished by  the  fact  that  an  alteration  to  muscovite  raises  the  specific 
gravity  of  the  orthoclase,  while  that  to  kaolin  lowers  it.  PL  XXIY. 
Fig.  6  represents  an  orthoclase  completely  altered  to  muscovite  (pini- 
toid).  Quartz  is  almost  always  mixed  with  these  pseudomorphs  in  vari- 
able amounts.  Moreover,  the  mass  becomes  penetrated  by  solutions 
carrying  iron,  manganese,  and  lime,  from  which  are  deposited  limonite, 
pyrolusite,  and  calcite.  Under  the  influence  of  accessory  solutions  the 
epidote  is  produced  which  is  so  often  present  in  decomposed  ortho- 
clase. 

In  the  so-called  pseudomorphs  of  cassiterite  after  orthoclase  from 
Huel  Coates  in  St.  Agnes  parish,  Cornwall,  tourmaline  and  quartz, 
besides  cassiterite,  form  a  principal  part  of  the  muscovite.*  The 
alteration  of  granite  to  greisen  must  be  ascribed  to  the  same  processes 
which  give  rise  to  these  pseudomorphs. 

*J.  Arthur  Phillips,  On  the  structure  and  composition  of  certain  pseudo- 
morphic  crystals  having  the  form  of  orthoclase.  Journ.  of  the  Chem  Soc  Aug. 
1875. 


TKLGLIflIC  MINERALS.  287 


MINEKALS  OF  THE  TBICLHSTIC  SYSTEM. 

THE  minerals  of  the  triclinic  or  asymmetric  system  are  chiefly  dis- 
tinguished by  negative  characteristics.  Sections  of  all  such  minerals 
are  unsymmetrical  in  all  zones  ;  the  same  is  true  of  all  figures  pro- 
duced by  intersecting  cleavages.  Each  cleavage  is  parallel  to  only  one 
face ;  hence  there  are  no  equivalent  cleavage  cracks  which  intersect 
one  another.  Cleavage  cracks  which  intersect  always  belong  to  crys- 
tallographically  dissimilar  faces.  In  general,  those  faces  parallel  to 
which  there  is  cleavage  are  made  the  pinacoids. 

The  triclinic  minerals  are  optically  biaxial ;  their  ellipsoid  of  elas- 
ticity is  triaxial,  but  is  different  for  each  wave-length.  Hence  there  is 
dispersion  of  the  optic  axes  and  of  all  three  axes  of  elasticity,  although 
these  dispersions  are  generally  small,  and  practically  may  be  neglected  in 
most  instances.  From  the  absence  of  all  symmetry,  there  is  no  definite 
relation  between  the  position  of  the  axes  of  elasticity  and  the  arbitrary 
co-ordinates,  which  are  chosen  as  crystallographic  axes.  In  general,  no 
uxis  of  elasticity  coincides  with  a  crystal  axis ;  when  this  is  approximately 
the  case  (oligoclase),  the  optical  behavior  resembles  that  of  a  monoclinic 
crystal,  as  far  as  concerns  the  extinction  angles  on  certain  faces.  In 
parallel  polarized  light  all  sections  which  are  not  cut  at  right  angles 
to  an  optic  axis  are  doubly  refracting,  and  between  crossed  nicols  ex- 
hibit the  quadruple  alternation  of  darkness  and  light.  The  direction 
of  extinction  is,  in  general,  inclined  to  the  crystal  outline,  to  the  cleav- 
age, and  to  the  diagonals  of  these  forms.  Sections  at  right  angles  to  an 
optic  axis  remain  uniformly  light  in  all  positions  between  crossed 
nicols,  and  exhibit  an  axial  figure  in  convergent  light,  whose  appear- 
ance is  analogous  to  that  of  an  orthorhombic  or  monoclinic  mineral. 

Sections  at  right  angles  to  a  bisectrix  give  an  interference  figure  in 
convergent  white  light  which  is  distinguished  from  that  of  an  ortho- 
rhombic  or  monoclinic  crystal  by  the  fact  that  the  distribution  of  the 
colors  is  unsymmetrical,  both  with  respect  to  the  trace  of  the  axial 
plane  and  to  one  normal  to  it,  as  well  as  unsymmetrical  to  the  centre 
of  the  axial  figure.  Thus  several  dispersions  occur  together  which 
would  be  distinguished  in  monoclinic  crystals  as  inclined,  horizontal, 
and  crossed.  The  optical  character  is  designated  as  positive  or  negative 
in  this  system  also,  according  to  whether  the  axis  of  least  or  greatest 
elasticity  bisects  the  acute  angle  between  the  optic  axes. 


288        PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

In  triclinic  minerals,  which  exhibit  pleochroism,  all  sections  are 
pleochroic  which  do  not  lie  at  right  angles  to  an  optic  axis.  The 
maximum  differences  of  color  are  90°  apart,  and  generally  coincide 
with  the  directions  of  extinction,  though  not  necessarily. 

Microdine. 

Literature. 

A.  DES  CLOIZEAUX,  Memoire  sur  1'existence,  les  proprietes  optiques  et  cristallo- 
graphiques,  et  la  composition  cliimique  du  microcline,  nouvelle  espece  de  feld- 
spath  triclinique  &  base  de  potasse,  suivi  de  remarques  sur  1'examen  microscopique 
de  1'orthose  et  des  divers  f  eldspaths  tricliniques.  Ann.  de  Chim.  et  de  Phys.  (5). 
IX.  1876.— Also  C.  R.  1876.  LXXXII.  885-891. 

Microcline  is  so  closely  related  to  orthoclase  in  habit  and  angles 
that  often  the  two  cannot  be  distinguished  crystallographically.  The 
angle  P/\M,  which  for  orthoclase  is  90°,  for  microcline  is  90°  16'-90° 
25'.  As  a  rock  constituent  microcline  never  forms  regularly  bounded 
crystals,  but  irregular  grains,  which,  however,  are  partly  bounded  by 
crystal  faces  when  they  project  into  cavities  of  the  rock  (as  in  many 
granites).  The  form  is  then  that  of  the  orthoclase  represented  in 
Fig.  98.  These  crystals  and  grains  are  scarcely  ever  simple  individuals, 
but  are  polysynthetic  masses,  composed  of  lamellae  and  stripes  arranged 
according  to  two  laws  of  twinning,  the  albite  and  pericline.  Moreover 
in  these  crystals  and  grains  the  microcline  is  more  or  less  intergrown 
with  orthoclase  and  albite.  The  dimensions  of  the  microcline  lamellae 
as  well  as  of  the  intercalated  orthoclase  and  albite  masses  are  almost 
always  microscopic.  On  the  faces  P  and  x  (the  faces  bear  the  same 
notation  as  for  orthoclase  with  the  modifications  necessitated  by  the 
triclinic  system)  the  double  twin  lamination  shows  itself  in  two  systems 
of  very  fine  striations,  one  of  which  is  parallel  to  the  edge  P  :  J/,  the 
other  is  normal  to  it,  or,  rather,  is  not  noticeably  inclined  to  it.  The 
albite  lamellae  are  intergrown  with  microcline  in  the  same  manner  as 
with  orthoclase,  and  often  give  the  face  J[f  a  distinctly  striated  appear- 
ance. Furthermore,  the  apparently' simple  microcline  crystal,  which  in 
reality  is  polysynthetic,  forms  twins  according  to  the  Carlsbad  and 
Baveno  laws. 

Microcline  cleaves  along  P  and  M  exactly  as  orthoclase ;  there 
is  an  imperfect  cleavage  parallel  to  the  left-hand  prism  <x>'P  (110) 
indicated  by  occasional  cracks.  The  position  and  inclination  of  the 
cleavage  cracks  in  the  different  zones  is  exactly  the  same  as  in  ortho- 
clase, since  the  slight  difference  in  the  angle  of  the  cleavage  faces 


MICROCLINE.  289 

Is  scarcely  or  not  at  all  noticeable.     In  microcline,  also,  the  cleavage 
cracks  are  only  perceptible  in  very  thin  sections. 

The  specific  gravity  =  2.56.  The  chemical  composition  and  chemi- 
cal reactions  are  the  same  as  for  orthoclase.  Hence  the  distinction 
betweer  microcline  and  orthoclase  lies  essentially  in  their  optical 
behavior. 

Microcline  becomes  transparent  and  colorless ;  the  index  of  refrac- 
tion and  strength  of  double  refraction  have  not  been  measured,  but  so 
far  as  fche  polarization  phenomena  can  be  relied  upon,  they  appear  to 
correspond  exactly  to  those  of  orthoclase.  The  position  of  the  axial 
plane  is  analogous  to  that  in  orthoclase,  but  is  not  absolutely  normal  to 
Jf,  making  with  this  face  an  angle  of  82°-83°  ;  its  trace  on  M  is  in- 
clined 5°-6°  to  the  edge  P :  M  in  the  direction  of  a  positive  orthodome. 
The  acute  axial  angle  is  88°-90°  in  oil.  The  obtuse  positive  bisectrix 
is  not  normal  to  M  as  in  orthoclase,  but  varies  15°  30'  K  K. 
from  this  normal.  The  dispersion  about  this  bisectrix  is 
P  <  v.  Therefore  cleavage  plates  or  sections  parallel  to 
P  and  M  in  polarized  light  behave  as  follows :  a  simple  M 
cleavage  plate  parallel  to  JP,  which  is  bounded  by  M  and  K 
(100),  as  in  the  left-hand  half  of  Fig.  106,  and  which  is  in  a 
the  conventional  crystallographic  position,  that  is:  has  the  F~is- 10G 
acute  edge  P:M  above  on  the  left,  becomes  dark  between  crossed 
nicols  when  the  directions  a  and  c,  the  bisectrices  of  the  angle  of  the 
optic  axes,  are  parallel  to  the  principal  sections  of  the  nicols.  In  other 
words,  the  direction  of  extinction  is  inclined  15°  30'  to  the  trace  of  the 
cleavage  parallel  to  M,  or  the  extinction  angle  on  P  is  positive  (cf. 
plagioclase),  that  is,  it  is  so  that  the  axis  of  elasticity  a  passes  from  the 
left  front  to  the  right  back,  when  the  crystal  is  properly  placed  above 
the  Tipper  basal  plane.  If,  now,  a  second  plate  parallel  to  P  be  placed 
in  twin  position  according  to  the  albite  law,  the  twinning  axis  normal 
to  J/~,  it  will  have  the  position  of  the  right-hand  half  of  Fig.  106,  and 
its  direction  of  extinction  will  be  inclined  15°  30'  to  the  trace  of  M, 
but  on  the  opposite  side.  The  sum  of  the  extinction  angles  in  the  two 
halves  of  the  twin  will  therefore  be  31°.  This  extinction  angle  of 
15°  30'  on  P  is  the  most  characteristic,  surest,  and  simplest  means  of  dis- 
tinguishing microcline  from  orthoclase.  Since  all  apparently  simple 
microcline  crystals  generally  consist  of  many  slender  lamellae  twinned 
after  the  albite  law,  a  cleavage  plate  parallel  to  P  exhibits  a  great 
number  of  differently  colored  stripes  between  crossed  nicols,  the  alter- 
nating stripes  having  the  same  color  when  of  the  same  thickness.  Each 
system  of  these  stripes  becomes  dark  when  their  longer  direction 
19 


290        PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


(parallel  to  M)  is  inclined  15°  30/  to  a  principal  section  of  the  nicols. 
But  nearly  all  microclines  are  also  twinned  polysynthetically  according 
to  the  pericline  law  ;  the  twinning  axis  is  1}.  Lamellae  arranged  accord- 
ing to  this  law  are  bounded  in  sections  along  P  by  lines  running 
parallel  to  the  edge  P :  K  (001 : 100)  (Fig.  107).  Since  the  angle  M :  K 
(010 : 100)  is  almost  exactly  a  right  angle  in  micro- 
cline, the  boundary  lines  of  the  lamellae  twinned  accord- 
ing to  the  pericline  law  are  normal  to  the  edge  P :  M, 
also  normal  to  the  boundaries  of  the  lamellae  twinned 
after  the  albite  law.  Hence  both  systems  of  lamellae 
intersect  at  right  angles.  The  twinning  axis  of  the  albite  law,  normal 
to  J/,  and  that  of  the  pericline  law,  #,  do  not  diverge  perceptibly  from 
one  another.  Consequently,  the  extinction  angles  in  each  system  of 
lamellae  coincide  with  one  another.  Between  crossed  nicols  sections 
along  P  exhibit  a  colored  rectangular  grating  or  plaid  (PL  XXV. 
Fig.  1),  in  which  there  are  always  two  sets  of  bars  perpendicular  to  one 
another  which  become  dark  at  the  same  time,  with  an  extinction  angle 
of  15°-16°.  This  striking  phenomenon,  which,  except  for  a  change  of 
angles,  is  the  same  in  all  sections  which  are  not  parallel  to  Jf,  immedi- 
ately distinguishes  microcline  from  all  other  feldspars. 

Both  systems  of  lamellae  often  reach  such  microscopic  dimensions 
that  it  is  no  longer  possible  to  determine  the  extinction  of  the  different 
lamellae  even  with  the  highest  rnagnifying-powers.  The  eye  then  only 
receives  a  general  impression  of  this  rectangular  grating.  The  differ- 
ent lamellae  very  rarely  attain  the  breadth  they  possess  in  ordinary 
lime-soda  feldspars;  occasionally  also  one  or  both  systems  of  lamellae  is 
wanting.  In  these  cases  the  characteristic  extinction  angle  on  P  (15°- 
16°)  is  always  the  means  of  distinction  from  other  feldspars. 

Usually  such  sections  parallel  to  P  when  between  crossed  nicols 
exhibit  irregularly  bounded  flakes,  which  are  dark  when  the  twinning 
boundaries  run  parallel  to  a  principal  section  of  the  nicols.  They  have 
straight  or  parallel  extinction,  and  belong  to  orthoclase.  In  the  same 
way  there  are  bands  which  run  nearly  or  exactly 
parallel  to  an  edge  P :  K,  less  frequently  to  an 
edge  P :  T  or  P :  Z,  and  exhibit  a  stronger 
double  refraction  than  microcline  and  orthoclase, 
and  show  themselves  finely  twinned  parallel  to 
the  face  J!/,  but  which  possess  an  extinction 
angle  of  about  4°.  They  belong  to  albite  (PL 
XXY.  Fig.  1). 

Cleavage  plates  parallel  to  M  would  have  the  axes  of  elasticity  a 


108 


MICROCLINE.  291 

and  B  in  the  position  shown  in  Fig.  308;  the  directions  of  extinction, 
then,  are  tlie  same  as  in  orthoclase,  and  microcHne  plates  parallel  to  this 
face  would  be  dark  between  crossed  nicols  when  the  cleavage  along  P 
makes  an  angle  of  5°  with  a  principal  section  of  the  nicols.  The  in- 
clination of  the  axis  of  elasticity  a  to  the  crystal  axis  lies,  as  in  ortho- 
clase, in  the  sense  of  a  positive  hernidome ;  it  is  positive  (cf.  plagioclase). 
Hence  microcline  sections  parallel  to  M  cannot  be  distinguished  from 
similar  sections  of  orthoclase  in  parallel  polarized  light,  and  inclusions 
of  the  latter  in  microcline  cannot  be  recognized  in  such  sections  in  this 
way.  Albite  stripes  in  % microcline  in  sections  along  M  run  nearly 
parallel  to  the  vertical  axis;  they  stand  out  because  of  their  stronger 
double  refraction,  and  consequently  higher  interference  colors,  and  ex- 
hibit a  different  extinction  (+ 18°  to  20°).  They  are  shown  in  PL 
XXV.  Fig.  2.  Occasionally,  there  is  another  system  of  bands  which 
are  inclined  about  16°-18°  to  the  vertical  axis,  and  whose  extinction 
lies  between  that  of  microcline  and  normal  orthoclase  and  that  of  albite, 
and  is  inclined  about  12°  to  a.  They  belong  to  another  feldspar,  which 
possesses  the  optical  orientation  of  the  schillerizing  orthoclase  of 
Frederiksvarn. 

In  convergent  light  plates  of  microcline  along  M  do  not  exhibit  the 
emergence  of  a  perpendicular  bisectrix,  as  in  orthoclase,  but  of  a  rather 
oblique  one.  On  the  margin  of  the  field  of  view  the  rings  belonging 
to  one  axis  are  noticeable,  the  axis  itself  being  situated  outside  of  the 
field. 

According  to  E.  Mallard  *  and  A.  Michel-Levy,  f  it  seems  highly 
probable  that  orthoclase  and  microcline  are  not  dimorphous,  but  identi- 
cal, since  they  proved  that  the  optical  behavior  of  orthoclase  would  be 
a  necessary  consequence  of  an  intimate  multiple  twinning  of  microcline 
lamellae  after  the  albite  and  pericline  law.  This  theory  is  strongly 
supported  by  the  fact  that  in  these  bodies  the  relative  cohesion  and  the 
specific  gravity  are  the  same  in  each,  while  these  properties  are  gener- 
ally different  in  heteromorphous  bodies. 

The  alteration  processes  of  microcline  are  exactly  the  same  as  those 
of  orthoclase. 

Microcline  occurs  with  orthoclase,  often  almost  completely  replacing 
it,  in  granites,  syenites,  elseolite  syenites,  and  gneisses.  The  feldspar 
of  so-called  graphic  granite  is  almost  always  microcline.  Microcline 
appears  less  frequently  in  quartz  porphyries  and  other  porphyries,  and 

*  Explication  des  phenom£nes  optiques  anomaux.     Paris,  1877.  103. 
f  Bull.  Soc.  Min.  Fr.  1879.  II.  135. 


292        PHYSIOGBAPHY  OF  THE  ROCK-MAKING  MINERALS. 

still  more  rarely  as  the  groundmass  of  these  rocks  becomes  microfelsitic 
or  glassy.  In  the  younger  eruptive  rocks  the  sanidine  very  rarely  ex- 
hibits a  structure  which  entitles  it  to  be  placed  under  microcline  '(cf. 
Sanidine). 

The  Group  of  Plagiodases. 
Literature. 

A.  DBS  CLOIZEAUX,  Memoire  sur  les  qualites  optiques  birefringentes  caracteristiques 
des  quatre  principaux  feldspaths  tricliniques  et  sur  un  precede  pour  les  distinguer 
immediatement  les  uns  des  autres.  Ann.  de  Claim,  et  de  Phys.  1875.  (5).  IV. 
and  C.  R.  1875.  LXXX.  36^371. 

—  Exanien  microscopique  de  1'ortnose  et  des  divers-  feldspatlis  tricliniques.     C.  R. 

1876.  LXXXIL  1017-1022. 

—  Nouvelles  recherches   sur   1'ecartement  des  axes  optiques,  ^orientation  de  leur 

plan  et  de  leurs  bissectrices  et  leurs  divers  genres  de  dispersion,  dans  1'albite  et 
1'oligoclase.     Bull.  Soc.  min.  Fr.  1883.  VI.  89-121. 

—  Oligoclases  et  andesines.     Ibidem.  1884.  VII.  249-336. 

E.  MALLAKD,  Sur  I'isoinorphisme  des  feldspatlis  tricliniques.     Bull.  Soc.  min.  Fr. 

1881.  IV.  103. 
G.  VOM  RATH,  Die  Zwillingsverwachsung  der  triklinen  Feldspathe  nach  dem  sog. 

Periklingesetz  und  liber  eine  darauf  gegrundete  Unterscheidung  derselben.     B. 

M.  1876.  Febr.  and  N.  J.  B.  1876.  689-714. 
M.  SCHUSTER,  Ueber  die  optische  Orientirung  der  Plagioklase.     T.  M.  P.  M.  1880. 

III.  117-284. 
—  Bemerkungen  zu    E.  MALLARD'S  Abhandlung    "Sur   risomorpkisme  des  feld- 

spatks   tricliniques."     Nachtrag    zur  optischen    Orientirung  der  Plagioklase. 

Ibidem.  1882. 'V.  189-194. 
G.  TSCHERMAK,  Die  Feldspathgruppe.     S.  W.  A.  1864.  December.  L. 

Under  plagiodases  are  here  included  the  lime-soda  feldspars,  that  is, 
albite  and  anorthite,  and  their  isornorphous  mixtures  from  the  albite, 
oligoclase,  andesine,  labradorite,  bytownite,  and  anorthite  series.  The 
chemical  composition  of  the  theoretical  albite  is  Na2GpAJ203,  6SiO2  = 
ISTa2,  A12,  Si.OJ6=' Ab ;  that  of  anorthite,  2CaO,  2A12O3, 4SiO2  =  Ca2,  A12, 
Ala,  Si4O16  =  An.  All  other  lime-soda  feldspars,  then,  are  isomorphous 
mixtures  of  albite  and  anorthite  =  Abn,  Anm.  Of  tli^-many  possible 
mixtures  certain  ones  occur  more  frequently,  and  have  received  par- 
ticular  names.  If  these  be  enlarged  by  the  addition  of  those  com- 
pounds closely  connected  with  them,  then,  following  Tschermak,  the 
lime-soda  feldspars  or  plagiodases  may  be  brought  into  the  following 
table : 

Albite  series  embraces  the  compounds  Abi,  An0    —    Ab8,  Ani 
Oligoclase  series     "          "  "  Ab6,  Ani     —    Ab2,  Ana 

Andesine  series      "          "  "  Ab3)  An2    —     Ab4,  An3 

Labradorite  series  "          "  "  Abi,  Ani     —    Abi,  An2 

Bytownite  series     "          "  "  Abi,  An3    —    Abi,  An6 

Anorthite  series     "          "  "  Abi,  Ang    —    Ab0,  Ant 


THE  PLAGIOCLASES. 


293 


In  petrography,  where  so  sharp  a  determination  of  the  proportions 
of  the  mixtures  in  many  cases  is  not  possible,  it  becomes  necessary  to 
unite  the  andesine  series  with  the  oligoclase  series,  and  the  bytownite 
series  with  the  labradorite  series,  and  to  speak  of  albite,  oligoclase, 
labradorite,  and  anorthite  as  the  plagioclases,  since  the  name  of  the 
feldspar  is  also  used  for  that  of  the  series.  There  has  been  also  in- 
eluded  under  the  term  plagioclase  in  petrography  a  number  of  feld- 
spars which  have  been  but  slightly  investigated,  and  which,  by  their 
small  percentage  of  CaO  and  high  percentage  of  K2O,  present  a  sepa- 
rate series  of  compounds,  if  they  do  not  resolve  themselves  into  very 
intimate  mechanical  mixtures.  The  following  statements  relate  exclu- 
sively to  plagioclases  proper,  or  lime-soda  feldspars : 

The  crystal  forms  of  the  plagioclases  exhibit  great  similarity  of 
habit  and  angle  measurements  among  themselves,  and  also  with  those 


is.  109 


of  orthoclase  and  microcline.  The  most  essential  difference  rests  in 
the  fact  that  the  angle  P/\Mis  not  90°,  but  lies  between  93°  36'  for 
albite  and  94°  10'  for  anorthite ;  there  are  also  certain  differences  of 
angle  in  the  inclinations  of  ihe  other  faces.  The  rock-making  plagioclases 
do  not  always  exhibit  crystal  boundaries,  but  are  very  often  massive. 
Well-developed  crystals  only  occur  in  rocks  possessing  a  clearly  marked 
porphyritic  structure.  They  are  then  bounded  principally  by  the  faces 
P  =  oP  (001),  M  =  oo  P  %  (010),  T=v>/P  (110),  I  =  oo JP/  (110), 
x  =  F  ,&>  (101),  y  =  fyP^  (201),  which  are  accompanied,  as  in  or- 
thoclase, by  the  subordinate  faces  n  =  %P;&  (021),  0  —  Pt  (111), 
v  =•  jP  (111)?  and  others.  The  habit  of  the  simple  crystals  is  some- 
times tabular  parallel  to  M  (Fig.  109),  sometimes  slender  prisms  par- 
allel to  a,  like  Fig.  99  for  orthoclase  ;  it  is  also  peculiarly  rhombic  (Fig. 
110)  in  certain  rocks  because  the  faces  P  and  M  are  wanting,  or  be- 
cause the  latter  is  but  slightly  developed.  The  angles  of  most  impor- 


294 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


tance  in  determining  the  cross-sections,  whose  forms  are  readily  derived 
from  the  figures  just  given,  are  the  following  for  albite,  and  but 
slightly  different  for  the  other  plagioclases  :  P/\M=  93°  36',  P/\T  = 
110°  50',  Pf\l  =  114°  42',  P/\x  =  52°  IT,  P /\y  =  97°  54',  T/\l  = 
120°  47',  T/\M=  119°  40',  l^M-=  119°  33'. 

Simple  crystals  are  comparatively  rare,  and  the  polysynthetic  twin- 
ning, which  is  the  most  important  outward  character  of  the  plagio- 
clases when  considered  macroscopically,  plays  just  as  important  a  role 
microscopicallly.  The  commonest  law  of  polysynthetic  twinning  in 
plagioclase  is  the  albite  law ;  the  twinning  axis  the  normal  to  J/,  com- 
position plane,  M.  Fig.  Ill  represents  a  simple  twin  of  this  kind 
having  a  pribinatic  habit,,  Fig.  112  represents  such  a  one  with  tabular 
habit  and  with  very  small  prism  faces.  In  this  kind  of  twinning  the  P 
faces  of  the  two  individuals  make  a  reentrant  angle  of  172°  48'  with 
one  another,  their  x  faces  one  of  172°  42',  and  in  the  prism  zone  similar 
prism  faces  adjoin  one  another.  Rock-making  plagioclases  are  char- 


31 


IIS 


JFig.  113 


.  114 


acterized  by  the  frequent  repetition  of  this  twinning,  so  that  a  crystal 
consists  of  a  great  number  of  thin  plates  parallel  to  M.  The  reentrant 
angles  between  the  P  faces  then  give  rise  to  the  well-known  twin 
striation  parallel  to  the  edge  P :  M  on  the  basal  plane  of  such  crystals. 
A  section  parallel  to  K  through  such  a  multiple  twin  would  have  the 
form  represented  in  Fig.  113 ;  in  a  section  which  is  parallel  or  inclined 
to  the  base  the  reentrant  angles  would  be  cut  off,  but  the  twinning 
planes  are  often  seen  quite  distinctly  by  transmitted  light,  especially 
when  the  section  is  inclined  to  P  and  the  boundaries  of  the  lamellae 
are  illuminated  obliquely  (Fig.  114).  The  twinning  must  be  visible 
in  all  sections  which  are  not  parallel  to  M. 

Much  more  rarely  the  twinning  is  according  to  the  pericline  law  ; 
the  twinning  axis  is  5,  the  composition  plane  parallel  to  the  rhombic 
section.  By  this  method,  when  it  is  repeated  polysynthetically, 
there  must  be  a  striation  on  the  face  M.  In  albite,  according  to  G. 
vom  Rath,  this  is  inclined  forward  13°  -  22°  less  than  the  edge 


THE  PLAGIOCLASES. 


295 


edge  P\M,  to  which  the  cleavage  is  parallel  (Fig.  115).  In  oligoclase 
the  angle  between  this  striation  and  the  edge  P :  M  is  only  4°  in  the 
same  direction,  in  andesine  0°,  in  labradorite  2°-9°  in  the  opposite 
direction,  that  is,  the  striations  are  inclined  more  steeply  forward  than 
the  edge  P:M;  for  anorthite  18°  in  the  last-named  direction.  The 
polysynthetic  twinning  after  the  pericline  law  not  infrequently  occurs 
in  combination  with  that  after  the  albite  law ;  twin  striation  is  then 
present  on  P  and  M.  Fig.  116  represents  a  crystal  bounded  by  P, 
M,  and  K  (100)  with  albite  and  pericline  lamellae.  On  the  basal  plane 
the  two  systems  of  lamellae  intersect  nearly  at  right  angles ;  the  crys- 
tallographic  axial  angle  y,  which  has  different  values  for  different 
plagioclases,  is  never  more  than  1°  from  a  right  angle.  Fig.  116  shows 
that  all  sections  through  such  a  polysynthetic  crystal  must  exhibit 
intersecting  systems  of  lamellae  whose  inclination  to  one  another  is 


no 


Fig. 


dependent   on  the  position   of  the   section.     The  lamination  is  only 
single  on  the  face  Jtf. 

Such  polysynthetic  individuals,  after  the  albite  or  pericline  law,  or 
after  both  together,  often  grow  together  according  to  laws  correspond- 
ing to  the  Carlsbad,  Baveno,  and  Manebacher  laws  in  orthoclase.  Fig. 
117  presents  a  Carlsbad  twin  of  two  twins  after  the  albite  law,  which 
is  a  very  frequent  occurrence.  It  is  evident  that  the  lamellae  on  a 
basal  section  cannot  all  belong  to  the  P  faces,  but  partly  to  P  and 
partly  to  x  faces,  which  is  important  in  considering  their  optical  be- 
havior. The  great  variety  which  is  introduced  into  the  twinning  of 
the  plagioclases  by  the  combination  of  these  laws  is  still  further  in- 
creased by  the  fact  that  the  lamellae  are  by  no  means  formed  with 
theoretical  regularity.  They  often  wedge  out  in  the  middle  of  the 
crystal,  change  their  breadth,  fork  and  branch,  throng  in  one  part  of 


296         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

the  crystal  and  fail  in  another ;  they  do  not  always  run  parallel  to  the 
twinning  plane,  but  show  by  their  boundaries  that  the  composition 
faces  may  be  quite  irregular.  Their  breadth  bears  no  relation  to  the 
size  of  the  compound  individual,  varying  quite  irregularly.  But  it 
appears  that  quite  broad  lamellae  in  the  embedded  and  rock-making 
plagioclases  are  chiefly  confined  to  the  more  basic  series. 

The  dimensions  of  plagioclase  crystals  vary  between  the  widest 
limits.  In  general,  they  seldom  reach  the  upper  limits  of  the  ortho- 
clases;  they  sink  to  microlitic  dimensions,  and  then  usually  form  very 
thin  prisms  parallel  to  the  edge  P\  M  (PI.  XXY.  Fig.  3),  the  so-called 
lath-shaped  plagioclases  or  plagioclase  microlites.  The  more  acid 
plagioclases  particularly  tend  to  the  prismatic  development  parallel  to 
the  edge  P:M.  In  other  cases  the  plagioclase  microlites  assume  a 
tabular  form  parallel  to  M\  they  are  then  occasionally  of  scarcely 
measurable  thickness,  and  sometimes  have  a  rhombic  outline  formed 
by  P  and  x  or  by  P  and  y  (like  the  face  M  in  Fig.  112),  sometimes 
an  appropriately  hexagonal  one  like  the  M  face  in  Fig.  109,  or  an 
irregularly  six-sided  one  from  P,  x,  and  y.  This  tabular  form  appears 
to  be  particularly  characteristic  of  the  microlites  of  basic  plagioclases. 

Actual  incipient  forms  of  growth  and  skeleton  crystals  are  not 
definitely  known. 

Anomalieg  of  crystallization  are  extremely  common  among  the 
plagioclases.  Thus  ruin-like,  indented  terminations  are  very  frequent 
in  the  larger  individuals,  as  shown  in  PI.  XXY.  Fig.  4;  it  almost  ap- 
pears as  though  small  completed  crystals  had  grouped  themselves 
together  to  form  a  compound  individual.  Through  chemical  corrosion 
originally  sharp-edged  crystals  have  become  more  or  less  rounded  to 
grains,  whose  original  form  can  only  be  surmised  from  the  zonal 
structure  or  the  arrangement  of  interpositions.  In  other  cases  there 
arise  "  bays"  or  pockets  of  greater  or  less  depth,  which  may  amount 
to  a  hollowing  out  of  the  crystal,  or  in  the  other  extreme  may  simply 
consist  of  a  slight  etching  of  the  crystal  face.  Besides  these  chemical 
deformations,  which  are  chiefly  confined  to  the  porphyritic  eruptive 
rocks,  there  are  in  eruptive  and  schistose  rocks  the  same  fracturings 
(PI.  XXIII.  Fig.  5)  as  those  described  for  orthoclase,  and  the  same 
marginal  fissurings  and  crushings  (PI.  IV.  Figs.  3  and  4) ;  further,  a 
bending  of  the  twin  lamellae  (PL  IV.  Fig,  6),  or  a  dislocation  of  the 
same  through  broken  and  faulted  individuals.  According  to  L.  van 
Werveke,*  it  is  very  probable  that  a  twin  lamination  may  arise  in 

*  N.  J.  B.  1883.  II.  97. 


THE  PLAGIOCLASES.  297 

plagioclases  through  the  forces  which  brought  about  these  mechanical 
deformations  (movement  in  the  magma  and  mountain  pressure).  Such 
mechanical  twin  lamellae  are  chiefly  characterized  by  the  fact  that 
their  extent  and  course  appear  to  depend  on  fracture  lines  in  the 
•crystal. 

Zonal  structure  is  extremely  frequent  in  the  plagioclases  of  all 
rocks,  excepting  in  those  of  later  generation  in  the  ground  mass  of 
porphyritic  rocks.  It  is  in  very  many  cases  simply  a  consequence 
of  repeated  interruptions  in  growth.  There  is  then  no  physical  differ- 
ence noticeable  in  the  behavior  of  the  kernel  and  of  the  different 
shells.  In  other  cases,  however,  a  zonal  structure  is  first  noticeable 
between  crossed  nicols  by  the  fact  that  the  extinction  does  not  take 
place  at  the  same  time  in  the  kernel  and  in  the  different  shells,  but  the 
kernel  and  shells  extinguish  light  in  azimuths,  sometimes  differing  by 
a  number  of  degrees.  So  far  as  experience  goes,  the  extinction  angles 
are  always  so  related  to  each  other  as  to  indicate  that  the  character  of 
the  kernel  is  more  basic  than  that  of  the  shells.  This  phenomenon  is 
explained  by  the  assumption  that  there  exists  an  isomorphous  lamina- 
tion, in  which  an  original,  basic,  central  crystal  is  surrounded  by  shells 
of  other  plagioclases,  which  gradually  become  more  and  more  acid.* 
Another  explanation  of  this  phenomenon,  which  is  shown  in  PL  XXV. 
Fig.  5,  is  given  by  A.  Michel-Levy  .f  .  He  considers  it  the  result  of  a 
subrnicroscopic  twin  lamination  after  the  albite  and  pericline  laws. 

The  rock-making  lime-soda  feldspars,  like  the  monoclinic  potash 
feldspars,  appear  in  two  kinds  of  habit :  In  the  granular  and  porphy- 
ritic, older,  massive  rocks  and  in  the  schistose  rocks  they  have  the  dull, 
cloudy  appearance  which  characterizes  orthoclase ;  in  the  younger 
eruptive  rocks  they  appear  glassy  and  colorless,  like  sanidine.  The 
latter  appearance  is  called  the  microtine  habit. 

The  plagioclases  cleave  along  the  faces  P  and  J/,  the  more  perfect 
cleavage  being  that  parallel  to  P.  Both  cleavages  show  themselves  in 
sufficiently  thin  sections  by  cracks,  which  resemble  those  of  orthoclase, 
except  for  their  inclination.  They  do  not  generally  show  themselves 
in  thicker  sections.  Cleavages  parallel  to  the  faces  Tand  I  are  but 
rarely  indicated  by  distinct  cracks.  The  parting  parallel  to  an  oblique 
face,  which  is  so  characteristic  of  sanidine,  seldom  occurs  in  the  plagio- 
clases. The  diagnostic  importance  of  the  cleavage  in  the  plagioclases 

*  C.  Hopfner,  Uber  das  Gestein  des  Mte.  Tajumbina  in  Peru.  N.  J.  B.  1881.  II. 
164-192. 

fC.  R.  1882.  XCIV.  93  and  178. 


298         PHYSIOGRAPHY  OF  THti  ROCK-MAKING  MINERALS. 

is  not  so  great  as  in  the  orthoclases,  since  the  twin  lamination  takes  its 
place  to  a  certain  extent  as  a  means  of  optical  orientation. 

All  plagioclases  become  transparent  and  colorless.  Their  indices 
of  refraction  are  nearly  equal  to  that  of  Canada  balsam,  and  somewhat 
larger  for  anorthite  than  for  albite.  There  is  no  direct  determination  ; 
biit  from  the  axial  angle  Des  Cloizeaux  determined  flft  =  1.537  for 
albite,  which  corresponds  to  the  indices  of  refraction  calculated  by 
Gladstone's  law,  which  for  anorthite  would  be  1.573.  The  double 
refraction  is  not  large,  but  is  always  greater  than  for  the  orthoclases, 
as  shown  by  the  interference  colors,  and  apparently  decreases  with  the 
percentage  of  lime.  A.  Michel-Levy  determined  on  anorthite  y  —  a  — 
0.013.  Little  is  known  concerning  the  true  position  of  the  optical 
constants,  with  the  exception  of  albite.  However,  the  numerous 
investigations  of  Des  Cloizeaux,  and  especially  of  M.  Schuster,  have 
completely  determined  the  behavior  of  cleavage  plates  and  sections 
parallel  to  the  faces  P  and  M  in  parallel  and  convergent  polarized 
light,  and  have  rendered  it  the  most  important,  surest,  and  quickest 
means  of  distinguishing  these  minerals. 

Since  in  triclinic  minerals  there  is  no  regular  relation  between  the 
position  of  the  optic  axial  plane  and  the  crystal  form,  the  extinction 
on  a  crystal  face  between  crossed  nicols  in  parallel  light  will  not 
generally  take  place  parallel  to  a  crystal  edge  or  to  the  trace  of  a 
cleavage  face,  but  will  make  an  angle  with  it.  If,  now,  a  simple 
plagioclase  crystal  (Fig.  109)  stands  in  the  conventional  crystallographic 
position,  so  that  the  end  face  is  inclined  toward  the  observer  and 
slopes  from  left  to  right,  the  acute  edge  P  :  JAvill  be  above  to  the  left, 
the  obtuse  edge  below  to  the  right.  On  a  plate  parallel  to  P,  the 
direction  of  extinction  nearest  to  the  edge  P  :  M  can  either  deviate 
from  this  line  so  that  its  trace  on  P  runs  in  the  direction  of  the  edge 
P  :  I  or  in  the  direction  of  the  edge  P  :  T.  The  deviation  in  the  first 
direction  is  called  positive,  that  in  the  second  negative.  In  the  same 
manner,  the  direction  of  extinction  on  the  right-hand  face  M  can 
either  deviate  from  the  edge  Jbf :  P,  so  that  it  runs  in  the  direction  of 
the  edge  M :  a?,  or  in  the  reverse  direction  ;  the  first  deviation  is  called 
positive,  the  second  negative.  All  statements  concerning  the  directions 
of  extinction  and  other  optical  constants  made  in  the  following  pages 
relate  to  the  upper  face  P  and  the  right-hand  face  Jf,  in  the  position 
of  Fig.  109. 

For  pure  albite,  the  extinction  angles  on  P  are  between  -f-  4°  and 
+  5°,  on  M  about  -|~  19° ;  for  an  oligoclase,  with  the  composition 
Aba An,  on  P+  10°  4',  on  M+  4°  36';  for  an  andesine,  AbsAna,  on  P— 


THE  PLAG10CLASES. 


299 


2°  12',  on  M  -  7°  58';  for  labradorite,  Ab, An,,  on  P  -  5°  10',  on  M  - 
16°;  for  bytownite,  A^An,,  on  P  —  17°  40',  on  M  —  29°  28';  for  pure 
anorthite,  on  P  —  37°,  on  M  —  36°.  These  relations  are  represented  in 
Figs.  118  and  119  (s  is  always  the  position  of  the  direction  of  extinc- 
tion), and  it  is  evident  that  the  extinction  angle  on  both  faces  assumes 
greater  negative  values  with  increasing  percentage  of  lime.  The  tran- 
sition from  positive  to  negative  extinction  takes  place  on  both  the  faces 
P  and  M  on  the  borders  of  the  oligoclase  and  andesine  series.  Thus, 


UK' 


010 


'  k 

t 

010 

\ 

«*                  as 

'JDESINE.               LABRADORITE. 

"Fig.  US 

there  is  a  particular  orientation  of  the  directions  of  extinction  on  both 
cleavage  faces,  corresponding  to  every  variety  of  composition.  These 
relations  have  been  carefully  investigated  experimentally  by  M.  Schus- 
ter, and  mathematically,  from  a  theoretical  standpoint,  by  E.  Mallard, 
and  the  striking  correspondence  between  their  results  leaves  no  doubt 
about  the  correctness  of  Schuster's  law — that  for  every  combination  of 
albite  and  anorthite  there  exists  a  certain  extinction  angle  on  the  faces 


ANDESINE.  LABRADORITE.  BYTOWNITE.  ANORTHITE. 

Fig.  119 


P  and  M,  which  is  dependent  on  the  amount  of  these  substances  in 
the  compound.  The  table  on  page  300  presents  the  relations  be- 
tween the  extinction  angles  and  the  compounds,  from  which,  when 
either  the  composition  or  the  extinction  angle  is  given,  the  other  may 
be  found. 

In  a  basal  section  of  a  plagioclase  between  crossed  nicols  the 
lamellae  twinned  after  the  albite  law  must  in  general  be  differently 
colored,  since  the  section  cuts  them  in  different  directions  with 
respect  to  their  ellipsoids  of  elasticity.  But  since  in  Figs.  113 
and  114  the  lamellae  marked  with  even  numbers  have  the  same 


300 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


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Inclination  of  the 
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THE  PLAGIOCLASE8.  301 

position  throughout,  and  those  marked  with  odd  numbers  have 
another  position,  then  all  the  even  lamellae  and  all  the  odd  la- 
mellae will  exhibit  two  sets  of  interference  colors.  The  result  is  a 
colored  lamination,  which  is  extremely  characteristic  of  the  plagio- 
clases  (Plate  XXV.  Fig.  6).  If  the  section  be  rotated  between 
crossed  nicols,  then  one  set  of  lamellae  will  become  dark  for  a  par- 
ticular inclination  of  the  boundary  lines  between  the  lamellae  (trace 
of  M)  to  the  left  or  to  the  right  of  a  principal  section  of  the  nicols, 
which  inclination  varies  with  the  chemical  composition  of  the  feldspar. 
If  the  section  be  now  rotated  through  the  same  angle  to  the  right  or 
left  of  the  twinning  line,  the  second  set  of  lamellae  would  become 
dark,  if  they  were  cut  parallel  to  the  face  P.  But  this  is  not  the 
case,  and  therefore  the  extinction  angle  of  the  second  set  of  laminae  is 
not  exactly  the  same  as  that  of  the  first.  But  the  difference  is  always 
small,  and  in  general  it  is  more  convenient  and  sufficiently  exact  to 
determine  the  extinction  angle  on  P^  by  rotating  the  section  between 
the  points  of  maximum  darkness  for  each  set  of  lamellae,  and  halving 
the  angle  so  obtained.  If  the  extinction  angles  should  be  the  same, 
right  and  left,  for  both,  it  would  show  that  the  section  was  not 
parallel  to  P,  but  normal  to  the  twinning  plane  M.  In  many  basal 
sections  or  cleavage  plates  of  plagioclase  there  are  lamellae  which  do 
not  exhibit  the  same  interference  colors  or  extinction  angles  as  the 
two  sets  of  lamellae,  although  they  appear  to  be  inserted  according  to 
the  same  twinning  law.  Such  lamellae  belong  to  a  set  twinned  accord- 
ing to  the  Carlsbad  law,  which,  as  Fig.  117  shows,  are  not  cut  parallel 
to  jP,  but  to  x.  There  will  also  be  two  sets  of  these  latter  lamellae, 
arranged  according  to  the  albite  law,  which  in  turn  extinguish  almost 
symmetrically  on  both  sides  of  the  twinning  plane.  Plate  XXV. 
Figs.  4  and  6  exhibit  these  relations  in  the  brightness  of  the  different 
lamellae.  Lamellae  twinned  after  the  pericline  law  would  cross  the 
albite  lamellae  nearly  at  right  angles,  and  would  furnish  two  sets  of 
lamellae,  each  of  which  would  extinguish  the  light  at  approximately  the 
same  time  as  the  sets  of  albite  lamellae,  since  both  twinning  axes  very 
nearly  coincide  (compare  microcline).  In  all  other  sections  not  parallel 
to  M,  the  different  sets  of  lamellae  will  always  be  differently  colored, 
and  will  extinguish  in  different  azimuths,  which  are  unsymmetrical  to 
the  twinning  plane.  Only  in  sections  lying  in  a  zone  at  right  angles 
to  M  will  the  extinctions  in  both  sets  of  lamellae  be  symmetrical  to  the 
twinning  plane.  When  pericline  and  albite  lamellae  occur  together, 
the  angle  between  the  lamellar  systems  in  irregular  sections  varies  with 
the  position  of  the  section ;  otherwise,  the  relations  remain  the  same 


302        PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

(Plate  XXYI.  Fig.  1).  The  presence  of  Baveno  twins  in  a  plagio- 
•clase  shows  itself  as  in  orthoclase,  through  the  occurrence  of  a  twinning 
boundary,  running  diagonal  to  P  and  M<  in  sections  parallel  or  inclined 
to  the  cross-section  (querflache)  (PL  XXYI.  Fig.  2). 

Sections  of  a  plagioclase  parallel  to  M  will  only  exhibit  twin  lami- 
nation when  there  are  lamellae  according  to  the  pericline  law.  Their 
boundary  will  be  inclined  to  the  cleavage  along  P,  according  to  the 
position  of  the  rhombic  section  for  the  particular  composition  of  the 
feldspar  (column  4  of  the  table  just  given),  or,  if  the  composition 
plane  is  the  base  (the  rarer  case),  it  will  be  parallel  to  this  cleavage. 
The  extinction  is  to  be  measured  from  the  cleavage  along  P. 

If  the  section  is  very  much  inclined  to  the  twinning  plane,  and  the 
lamellae  are  very  thin,  it  may  happen  that  a  complete  extinction  does 
not  take  place.  It  is  due  to  the  fact  that  within  the  thickness  of  the 

section  two  wedge-shaped  lamellae  are 
superimposed.  The  conditions,  then, 
are  those  described  on  page  62. 

All  plagioclases  from  albite  to  anor- 
thite  in  convergent  polarized  light  show 
a  positive  bisectrix  more  or  less  inclined 
to  the  face  M.  The  axial  angles  about 
this  positive  bisectrix  vary  in  the  neigh- 
borhood of  90° ;  they  are  acute  for  albite 
with  p  <  v,  obtuse  for  oligoclase  with 
p  <  v9  acute  for  labradorite  with  p  >  v 
and  obtuse  for  anorthite  with  the  same 
dispersion.  The  size  of  the  axial  angle 

changes  for  different  light,  and  diminishes  in  a  complicated  ratio  with 
the  percentage  of  anorthite. 

The  approximate  position  of  the  axial  plane  is  best  understood 
from  the  projection  on  M  (010),  Fig.  120,  taken  from  Schuster's  work. 
The  positive  bisectrix  for  all  plagioclases  lies  very  nearly  in  the  plane 
of  the  zone  P  :  M  (001  :  010),  but  is  inclined  on  the  right-hand  M  face 
toward  the  acute  edge  P :  M  for  albite,  rights  itself  with  increasing 
anorthite  percentage  so  that  in  the  normal  oligoclases  it  is  slightly 
inclined  toward  the  obtuse  edge  P :  M,  and  this  inclination  increases 
more  and  more  with  the  labradorites,  bytownites,  and  anorthites.  In 
certain  oligoclase-albites  the  positive  bisectrix  very  nearly  coincides 
with  the  normal  to  ^M.  With  this  rising  up  of  the  point  of  emergence 
of  the  positive  bisectrix  there  is  combined  a  rotation  of  the  axial  plane 
so  that  the  negative  bisectrix,  which  in  albite  emerges  from  the  macro- 


THE  PLAGIOCLASES.  303 

pinacoid,  in  anorthite  appears  to  be  turned  about  70°,  and  leaves  the 
crystal  in  the  neighborhood  of  the  right  lower  front  corner.  The  in- 
clination of  the  positive  bisectrix  downward  from  the  normal  to  M 
(010)  is  about  18°  in  albite,  the  inclination  upward  for  anorthite  about 
42°. 

Cleavage  plates  and  sections  of  albite  parallel  to  M  in  convergent 
polarized  light  show  the  emergence  of  a  positive  bisectrix  to  one  side  of 
the  field  of  view  (for  the  proper  position  of  the  right-hand  M  face 
downward).  The  axes  themselves  do  not  come  within  the  field,  but 
their  outer  rings  are  equally  distinct  on  both  sides  when  the  convergence 
of  the  light  is  sufficient  and  the  plate  is  not  too  thin.  The  dispersion 
is  inclined  and  slightly  horizontal.  There  is  no  axial  figure  on  P. 

For  oligoclase  a  bisectrix  emerges  nearly  normal  to  Jkf,  the  inclina- 
tion being  toward  the  obtuse  edge  P  :  M.  The  dispersion  is  very 
slightly  inclined  and  slightly  crossed.  There  is  no  axial  figure  on  P. 

Labradorite  and  bytownite  show  curves  and  an  axial  bar  on  the 
right-hand  M  face,  which  indicates  that  an  axis  emerges  outside  of  the 
field  of  view  below  to  the  left.  Plates  parallel  to  P  show  the  same 
phenomenon,  but  for  proper  crystallographic  position  the  axis  emerges 
outside  of  the  field  above  to  the  right.  The  dispersion  is  distinctly 
crossed  and  slightly  inclined. 

Anorthite  plates  on  the  right-hand  M  face  show  an  axis  within  the 
field  not  far  from  the  margin  and  below,  and  on  the  upper  P  face  an 
axial  figure  within  the  field  and  back.  There  is  no  distinct  dispersion 
of  the  bisectrices. 

From  the  foregoing  it  is  clear  that  it  is  possible  to  determine  the 
proportions  of  a  mixture  within  certain  limits  which  depend  on  the 
perfection  of  the  material,  its  freshness,  and  not  too  complicated  twin- 
ning structure.  The  difficulty  lies  in  the  determination  of  the  character 
of  the  extinction  angle  in  the  cleavage  plate  or  thin  section  under  in- 
vestigation. They  are  diminished  by  combining  certain  extinction 
angles  on  P  with  those  on  M.  Small  extinction  angles  on  both  faces 
indicate  oligoclase  or  andesine,  and  it  is  generally  impossible  to  dis- 
tinguish between  these  unless  the  crystals  in  question  are  measurable. 
Large  extinction  angles  on  both  faces  characterize  bytownite  and 
anorthite.  Medium  extinction  angles  on  P  and  M  occur  in  albite  and 
labradorite.  In  order  to  distinguish  batween  the  last-named  varieties 
plates  parallel  to  M  are  used.  If  cleavage  cracks  parallel  to  the  prism 
are  present  in  ordinary  light,  the  character  of  the  extinction  is  easily 
told.  It  is  negative  when  it  lies  in  the  acute  angle  between  the  cleavage 
cracks  parallel  to  the  prism  and  base,  positive  when  in  the  obtuse  angle 


304        PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

between  these  cleavages.  If  the  cleavage  is  wanting,  convergent  light 
is  used.  In  albite  a  positive  bisectrix  emerges  almost  normal  to  Jtft 
but  in  labradorite  M  shows  no  recognizable  bisectrix  and  no  axis.  As 
between  anorthite  and  bytownite,  the  former  shows  an  axial  figure  on 
M  in  convergent  light,  which  is  situated  in  the  margin  of  the  field  of 
view ;  in  bytownite  the  axis  is  no  longer  in  the  field. 

For  the  correct  determination  of  cleavage  plates  they  should 
be  bounded  by  two  plain,  smooth  cleavage  faces.  If  they  have  but 
one  such  face,  they  should  be  secured  by  this  one  and  a  second  face 
ground  parallel  to  it.  If  the  material  is  too  fine-grained  to  furnish 
such  cleavage  plates,  unstriated  sections  should  be  sought  out  in  the 
thin  section,  whose  outlines,  if  possible,  are  evidently  those  of  JH,  and 
these  tested  in  parallel  and  convergent  light.  It  is  possible  in  this  way 
to  arrive  at  a  conclusion  as  to  the  approximate  basicity  of  the  plagio- 
clase,  and  in  particularly  good  cases  to  determine  it  accurately.  If  the 
plagioclase  is  greatly  twinned  after  the  pericline  law,  the  M  faces  are 
recognized  with  less  certainty,  and  the  determination  is  made  more 
difficult  if  not  impossible.  In  such  cases,  when  good  cleavage  pieces 
cannot  be  had,  a  sort  of  statistical  process  may  be  employed  which  has 
been  specially  elaborated  by  A.  Michel-Levy.*  It  is  evident  that  sec- 
tions of  a  plagioclase  at  right  angles  to  the  twinning  plane  M  can 
always  be  recognized  by  the  fact  that  the  extinctions  in  alternate 
lamellae  are  symmetrical  to  the  twinning  plane  M.  These  extinction 
angles  when  measured  have  very  different  values,  but  for  each  plagio- 
clase a  maximum.  For  microcline  it  is  18° ;  for  albite,  15°  45' ;  for 
oligoclase,  18°  30';  for  labradorite  under  certain  suppositions.  31°  15'; 
for  anorthite,  over  37°  21'.  Thus  it  is  evident  that,  for  example,  the 
occurrence  of  symmetrical  extinction  angles  of  25°  would  indicate  that 
the  feldspar  was  not  albite  nor  oligoclase,  but  a  distinction  between 
labradorite  arid  anorthite  would  not  be  possible.  In  general,  this  pro- 
cess is  not  applicable  unless  it  is  certain  that  the  maximum  extinction 
has  been  observed.  When  this  cannot  be  assumed,  such  a  determina- 
tion should  be  employed  with  the  greatest  caution. 

For  the  determination  of  plagioclase  microlites  A.  Michel  Levy 
proposed  to  employ  the  zone  P :  M,  in  which  they  are  developed  pris- 
matically,  so  that  their  longitudinal  axis  corresponds  to  the  axis  of  this 
zone.  In  this  zone  the  extinction  angles  of  a  lamella  measured  from 
the  zonal  axis  vary  in  microcline  from  0°  to  16° ;  in  albite,  from  0°  to 
19°  ;  in  oligoclase,  from  0°  to  2°  ;  in  labradorite  from  0°  to  17°,  or  from 

*  Ann.  des  Mines.  (7),  XII.  451. 


TUB  PLAGIOCLASES.  305 

0°  to  27°,  according  to  the  size  of  the  axial  angle  2  F;  in  anorthite, 
from  0°  to  37°.  From  this  it  is  seen  that  oligoclase  microlites  are 
well  characterized  by  the  fact  that  they  extinguish  light  almost  parallel 
to  their  length.  The  frequent  recurrence  of  extinction  angles  over  27° 
would  show  the  presence  of  anorthite.  Further  than  this  these  data 
cannot  be  used. 

In  cases  where  the  optical  determination  of  the  plagioclases  is 
impracticable,  their  specific  gravity  maybe  used  to  advantage.  It  may 
be  determined  on  small  grains  by  suspending  them  in  a  heavy  solution 
whose  density  has  been  determined  by  one  of  the  methods  described  on 
page  104,  or  by  taking  it  during  the  mechanical  separation  of  the  min- 
eral constituents  immediately  before  and  during  the  settling  of  the 
plagioclase  powder.  Tschermak  first  showed  that  the  specific  gravity 
of  the  plagioclases  increases  with  the  percentage  of  anorthite  in  such  a 
manner  that  it  can  be  calculated  for  a  particular  plagioclase  from  its 
relative  composition.  For  that  purpose  it  was  assumed  to  be  2.624  for 
pure  albite,  and  2.758  for  pure  anorthite.  Y.  Goldschmidt*  made  a 
large  series  of  determinations  on  feldspars,  which  average  somewhat 
lower  than  the  values  given  by  Tschermak,  although  the  differences 
are  not  great.  Barwald  determined  the  specific  gravity  on  the  ideally 
pure  albite  of  Kasbek  at  2.618.  In  the  following  table  the  values 
given  by  Tschermak  and  Goldschmidt  are  correlated : 

Sp.  gr.  according  Sp.  gr.  according  Typical  aver- 

to  Tschermak.  to  Goldschmidt.  age  ralue. 

Orthoclase    )  ..2.56-2.57  250-2.59  2.57 

Microcline    ) 

Albite 2.62-2.64  2.61-2.63  2.62 

Oligoclase 2.64-2.66  2.62-2.65  .        2.64 

Andesine 2.66-2.69  265  2.65 

Labradorite 2.69-2.71  2.68-2.70  2.69 

Bytownite .2.71-2.74  2.70-2.72  2.71 

Anorthite 2.74-2.76  2.73-2.75  2.75 

From  the  great  exactness  with  which  the  density  of  a  heavy  solu- 
tion can  be  regulated,  this  determination  of  the  feldspars  is  very  reliable, 
so  long  as  the  material  is  pure  and  fresh.  This  certainty  is  consider- 
ably lessened  by  the  presence  of  interpositions  as  well  as  by  alteration 
processes  in  the  feldspars  whose  specific  gravity  is  to  be  determined. 
It  is  to  be  remembered  that  the  commonest  inclusions  of  the  feldspars 
in  porphyritic  rocks  (gas  and  glassy  parts  of  the  magma)  diminish  the 
specific  gravity,  while  the  individualized  inclusions  of  the  feldspars 


*N.  J.  B.    B.-B.  I.  1880. 


306          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

in  granular  rocks  tend  to  increase  it.  The  influence  of  alteration  pro- 
cesses is  less  easily  foreseen,  because  the  alteration  products  are  difficult 
to  recognize  with  certainty.  A  kaolinization  and  zeolitization  would 
diminish  the  density;  the  development  of  carbonates,  the  formation  of 
mica  and  saussurite,  must  increase  it. 

The  chemical  investigation  of  the  feldspars  in  cleavage  plates  or  in 
powder  after  its  isolation  should  be  used  to  control  the  results  found  in 
other  ways.  Without  regard  to  the  fact  that  anorthite  and  bytownite 
are  decomposed  by  boiling  hydrochloric  acid  with  the  formation  of 
gelatinous  silica,  while  the  other  feldspars  are  not  attacked  at  all  or 
only  very  slightly,  they  are  distinguished  with  the  greatest  certainty  by 
Bo?icky's  method  from  the  amount  of  potassium,  sodium,  and  calcium 
fluosilicates.  The  advantages  of  spectrum  analysis  also  should  be 
borne  in  mind. 

The  alteration  processes  of  the  plagioclases  are  partly  the  same  as 
those  of  orthoclase  ;  sometimes  kaolin,  sometimes  muscovite,  or  perhaps 
paragonite,  is  formed.  From  the  nature  of  things,  calcite  is  more  com- 
mon besides  quartz  as  side  products  in  these  processes.  Zeolitization  is 
particularly  frequent  when  the  plagioclases  are  associated  with  nephe- 
line  or  minerals  of  the  sodalite  group  in  younger  eruptive  rocks,  but 
also  occurs  in  certain  rocks  of  the  diabase  and  gabbro  families.  The 
so-called  saussurite  alteration  of  the  basic  plagioclases  (labradorite,  by- 
townite, and  anorthite)  is  chiefly  confined  to  dynamo-metamorphic 
regions  ;  they  are  converted  into  an  aggregate,  which  consists  principally 
of  epidote  or  zoisite,  with  which  scapolite  is  occasionally  associated, 
while  the  soda  gives  rise  to  the  formation  of  albite.  The  alteration  of 
feldspars  to  pseudophitic  substances  has  rather  the  character  of  a  local 
process ;  it  has  been  observed  in  granular  limestones.  The  lime  and  alka- 
lies of  the  feldspars  must  have  been  replaced  by  magnesia  and  protox- 
ide of  iron  from  associated  minerals. 

Albite  has  a  greater  distribution  in  rocks  than  was  formerly  sup- 
posed. In  the  massive  non-glassy  condition  it  is  a  constituent  of  certain 
granitic  rocks,  its  occurrence  in  which  up  to  the  present  time  has  been 
investigated  but  little.  The  crystalloids  of  albite  exhibit  the  normal 
polysynthetic  twinning  of  the  granitic  plagioclases.  In  the  form  of 
microperthitic  intergrowths  with  orthoclase  and  microcline,  albite  is 
quite  generally  present  in  granites  and  gneisses,  especially  in  those 
with  high  percentage  of  silica.  It  is  very  probable  that  albite  is  some- 
times very  abundant  in  the  microcrystalline  gronndmass  of  porphyries 
and  porphyrites,  and  is  confounded  with  orthoclase  on  account  of  the 
lack  of  twinning.  Its  presence  is  rendered  quite  certain  by  the  chemi- 


THE  PLAGIOCLASES.  307 

« 

«al  composition  of  tlie  ground  mass  of  such  rocks,  but  it  has  not  been 
directly  proven  as  yet.  As  long  prismatic  microlites  of  microtine 
habit  it  occurs  in  the  groundmass  of  acid  trachytic  and  andesitic  erup- 
tive rocks,  and  probably  it  is  not  infrequent  among  the  porphyritic  se- 
cretions. Here  also  the  evidence  has  been  derived  mainly  from  the 
chemical  composition.  Albite  has  a  distribution  in  the  Archaean  rocks 
which  was  formerly  quite  overlooked,  especially  in  those  whose  crys- 
talline condition  has  been  brought  about  by  dynamo-metamorphic  pro- 
cesses. Thus  it  has  been  described  by  A.  Bohm  *  in  distinctly  poly- 
synthetic  grains  from  gneiss  in  the  Wechselgebirge,  the  north-eastern 
extension  of  the  central  range  of  the  Alps.  In  the  sericite  gneisses, 
phyllite  gneisses,  feldspar  phyllites  and  porphyroids,  it  sometimes  forms 
more  or  less  distinct  crystals,  at  other  times  grains,  which  occur  like 
porphyritic  sections ;  sometimes  intimately  associated  with  quartz 
and  muscovite,  it  forms  more  or  less  fine-grained  aggregates.  When 
very  fresh  it  is  white,  and  often  cloudy  to  dark  gray  from  abundant 
inclusions  of  carbonaceous  matter,  rutile  needles,  minute  fluid  and  gas 
interpositions ;  it  is  also  reddish  from  infiltrations  of  hydroxide  of 
iron.  Not  infrequently  the  twinning  is  entirely  absent,  or  there  are 
simple  twinned  halves  in  whose  separate  individuals  very  small  lamellge 
are  occasionally  inserted  in  twinned  position.  The  twinning  boundary 
is  often  a  very  irregular  face.  Finally,  albite  occurs  with  very  similar 
habit,  but  generally  in  much  smaller  grains  in  the  adinoles  of  diabase 
contact  zones  and  in  many  so-called  green  schists,  as  well  as  in  quartz 
nodules  and  veins  in  phyllites  and  clay-slates. 

Oligodase  in  massive  grains  and  crystals  is  one  of  the  most  frequent 
feldspars  in  granites,  syenites,  diorites,  and  their  porphyritic  equivalents, 
and  particularly  accompanies  orthoclase.  The  inclusions  and  structure 
are  exactly  the  same,  and  have  the  same  arrangement  as  in  the  ortho- 
clase of  the  same  rock,  with  which  it  is  frequently  intergrown.  When 
a  form  can  be  made  out  it  has  the  more  equidimensional  to  tabular  habit 
of  Figs.  109  and  117.  The  twin  lamination  is  seldom  if  ever  absent, 
and  the  lamellae  are  not  very  broad.  Oligoclase  occurs  in  the  same 
form  and  with  the  same  rnicrostructure  in  gneisses.  The  weathering 
leads  to  the  formation  of  kaolin  and  light-colored  mica,  with  an  acces- 
sory secretion  of  calcite  and  epidote.  In  the  diabase  rocks  and  their 
porphyritic  varieties  the  habit  of  the  oligoclase  is  generally  lath-shaped, 
with  P  and  M  equally  developed.  In  this  group  of  rocks,  even  when 
granular,  the  oligoclase  occasionally  contains  glass  inclusions.  The  pe- 

*  T.  M.  P.  M.  1883.    5.   202. 


308         PHYSIOGRAPHY  OF  TEE  ROCK-MAKING  MINERALS. 

culiar  crystals  bounded  chiefly  by  T,  I,  y  (Fig.  110),  occurring  in  the 
so-called  rhombic  porphyry,  belong  to  oligoelase,  according  to  O. 
Miigge.* 

Oligoclase  with  microtine  habit  forms  one  of  the  principal  constitu- 
ents of  trachytic  and  andesitic  rocks,  and  when  occurring  as  porphyritic 
secretions  has  chiefly  a  tabular  form  ;  as  a  constituent  of  the  ground- 
mass,  it  has  a  lath-shaped  form.  It  is  particularly  characterized,  like 
all  the  plagioclases  of  these  rocks,  by  an  abundance  of  glass  inclusions, 
which  are  often  scattered  through  it  like  a  net  (PL  XXYI.  Fig  3),  are 
often  arranged  zonally,  peripherally,  or  centrally,  and  are  rarely  isolated 
or  irregularly  arranged.  In  the  basaltic  rocks  the  oligoclases  are  mostly 
lath-shaped. 

Sunstone  is  the  name  applied  to  certain  oligoclases,  which  have  a 
beautiful  red  sheen  from  the  interposition  of  lamellae  of  specular  iron. 
The  familiar  occurrence  at  Twedestrand  has  been  investigated  micro. 
scopically  by  Th.  Scheerer.f  The  lamellae  of  specular  iron  lie  chiefly 
along  the  faces  P,  M,  and  a  prism,  in  part  also  parallel  to  a  face 


Andesine  has  exactly  the  same  geognostic  position  and  development 
of  forms  as  oligoclase  in  the  older  and  younger  eruptive  rocks  and  in 
the  gneisses. 

Labradorite  appears  to  be  confined  to  the  more  basic  eruptive  rocks 
and  to  certain  Archaean  rocks  rich  in  amphibole  and  pyroxene  ;  it  always 
appears  to  avoid  the  proximity  of  orthoclase  and  quartz.  The  massive 
labradorite  of  the  older  granular  massive  rocks  of  the  diorite  family 
possesses  the  same  habit  as  oligoclase  and  andesine  ;  the  same  is  true  in 
general  of  the  lath-shaped  labradorite  of  diabase  and  ophite.  On  the  other 
hand,  the  spathic  labradorites  of  the  gabbro  and  norite  series  are  often 
distinguished  by  peculiar  gray  or  grayish  brown  to  reddish  brown  colors, 
which  arise  from  interpositions,  which,  in  spite  of  all  differences  of  form, 
appear  to  belong  essentially  to  iron-ores  and  titaniferous  iron-ores. 
Long  opaque  or  brownish  translucent  plates  of  hexagonal,  rhombic,  or 
irregular  outline  are  particularly  characteristic,  and  are  probably  lim- 
onite  and  specular  iron.  Moreover,  there  are  also  acicular  microlites 
which  are  mostly  straight,  but  are  also  curved  and  bent  or  separated  into 
points.  In  many  labradorites  of  the  gabbros  and  norites,  partly  also 
in  the  ophites,  these  interpositions  sink  to  the  finest  dust-like  forms, 


*N.  J.  B.  1881.  II.  107  sqq. 

f  Pogg.  Ann.  1845.  LXIV.  (153.)    cf.  Isaac  Lea,  Proc.   Acad.  Nat.  Sci.   Phil. 
1866.  110. 


THE  PLAGIOCLASES.  309 

not  resolvable  even  with  the  highest  powers.  Moreover,  these  labra- 
dorites often  contain  microlites  of  pyroxene  and  hornblende,  crystals 
and  grains  of  associated  minerals,  and  quite  frequently  fluid  inclusions. 
Labradorite  occurs  with  the  same  habit  in  many  amphibolites  of  the 
Archaean  which  are  evidently  dynamo-metamorphic  gabbros. 

To  this  group  of  labradorites  belong  those  from  St.  Paul's  Island, 
Ojamo,  and  the  neighborhood  of  Kiew,  which  are  well  known  on 
account  of  their  beautiful  play  of  colors  and  their  broad  cleavage,  and 
whose  interpositions  and  microstructure  have  been  carefully  inves- 
tigated. Vogelsang  was  the  first  to  refer  the  iridescence  of  these 
labradorites  to  their  orderly  arranged  interpositions.  These  interposi- 
tions differ  from  those  of  ordinary  gabbro  labradorites  only  in  the 
beauty  of  their  development  and  in  their  usually  very  regular  arrange- 
ment parallel  to  the  vertical  and  brachydiagonal  axes.. 

The  tendency  of  these  gabbro  labradorites  to  the  simultaneous  de- 
velopment of  albite  and  pericline  twin  structure  is  to  be  noted,  as  well 
as  the  rarity  of  the  formation  of  carbonates  in  the  processes  of  de- 
composition, which  almost  always  lead  to  the  formation  of  saussurite. 
The  description  of  the  alteration  processes  which  take  place  with  the 
aid  of  solutions  arising  from  the  associated  minerals  (pyroxene,  olivine, 
ilmenite)  belongs  to  the  petrographical  part  of  this  work. 

The  labradorites  of  the  older  porphyritic  eruptive  rocks  (porphyrite, 
angite  porphyrite,  melaphyre,  etc.)  as  well  as  of  the  younger  volcanic 
rocks  (trachyte,  andesite,  basalt,  and  tephrite)  exhibit  exactly  the  same 
development  of  forms  and  microstructure  as  the  more  acid  feldspars  of 
the  same  rocks. 

Bytownite  possesses  the  same  geological  position,  the  same  devel- 
opment of  forms,  and  the  same  microstructure  as  labradorite  in  the  older 
and  younger  granular  and  porphyritic  rocks.  F.  Zirkel  *  has  shown 
that  the  occurrence  which  gave  the  name  to  this  series  of  plagioclases, 
lying  between  labradorite  and  anorthite,  is  a  mixture. 

Anorthite  occurs  in  granular  individuals  or  broad  tabular  aggre- 
gates in  a  few  diorites,  in  lath-shaped  forms  in  occasional  diabases 
and  in  the  teschenites,  in  large  spathic  masses  in  gabbro  and  norite, 
especially  in  the  olivine-bearing  varieties,  and  here  possesses  the  struct, 
ure  of  labradorite.  It  forms  tabular  crystals  in  the  most  basic  porphy- 
rites.  The  microtine  form  of  anorthite  is  found  in  many  andesites  and 
basalts,  especially  in  the  older  granular  segregations  in  these  rocks, 
which  occasionally  reach  the  surface  as  bombs,  or  lie  like  inclusions  in 

*  T.  M.  M.  1871.  61. 


310          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

the  lava-flows.  In  the  Archaean  rocks  labradorite,  bytownite,  and 
anorthite  are  only  found  in  amphibolites,  which  were  probably  once 
gabbros. 

Fischer*  has  investigated  the  basic  plagioclases  belonging  to  anor- 
thite and  bytownite  which  have  been  named  amphodelite,  latrobite, 
indianite,  rosellan,  poly argite  and  pyrholite,  and  the  pseudophitic  altera- 
tion of  the  same.  Des  Cloizeaux  (1.  c.)  carried  through  their  optical 
investigation.  Y.  Lasaulxf  and  Liebisch  J  described  the  feldspathic 
mixture  saccharite,  which  forms  nests  and  seams  in  serpentine. 

1.  Appendix. — Besides  the  true  plagioclase  group,  including  albite 
and  anorthite  with  their  isomorphous  mixtures  with  the  general  formula 
AbnAnm,  and  whose  members  never  possess  any  considerable  propor- 
tion of  the  compound  Or  =  K2Al,Si6O16,  there  appears  to  be  series  of 
triclinic  potash-soda  feldspars  whose  percentage  of  An  =  CaaAl2Al,Si40J6 
never  exceeds  a  certain  limit.  This  group  has  not  been  definitely 
known  until  recently,  and  its  occurrence  and  properties  have  been  but 
little  studied.  Feldspars  belonging  to  this  group  from  the  island  of 
Pantelleria  were  first  described  by  H.  Forstner  §  as  soda  orthoclasesr 
and  considered  monoclinic.  C.  Klein  ||  recognized  their  triclinic 
nature,  as  well  as  that  of  a  similar  feldspar  from  Hohenhagen,  and 
placed  them  near  oligoclase.  W.  C.  Brogger  T  then  found  in  the  augite 
syenites,  whose  intergrown  orthoclase  and  albite  have  already  been 
mentioned,  feldspars  which  showed  no  mechanical  mixture  of  ortho- 
clase and  albite,  and  behaved  optically,  in  part  monoclinic,  in  part  tri- 
clinic. Their  chemical  composition,  which  was  the  same  in  both  cases, 
indicated  that  they  were  isomorphous  mixtures  of  potash  and  soda  feld- 
spars, with  an  inconsiderable  percentage  of  lime  feldspar.  More  re- 
cently, H.  Forstner**  has  investigated  the  feldspars  of  Pantelleria  anew, 
and  has  determined  a  considerable  number  of  such  triclinic  potash-soda 
feldspars  with  small  percentage  of  lime,  chemically,  crystallographi- 
cally,  and  optically. 

*  Kritische,  mikroskopisch-mineralogische  Studien.    1.  Fortsetzung.  Freiburg,  i. 
Br.  1871.  40.  sqq. 

f  N.  J.  B.  1878.  623. 

\  Z.  D.  G.  G.  1877.  XXIX.  735. 

§  Ueber  Natronorthoklas  von  Pantelleria.    Z.  X.  1877.  I.  547. 

1  Ueber  den  Feldspath   im  Basalt  vom  Hohen  Hagen  bei  Gottingen   und   seine 
Beziehung  zu  dem  Feldspath  vom  Mte.  Gibele.     Gottinger  Nachrichten  1878  No  14 
and  N.  J.  B.  1879.  518. 

f  Die  silurischen  Etagen  2  und  3  im  Christiania-Gebiet.    Christiania.  1882.  260 
sqq.  and  293  sqq. 

**  Ueber  die  Feldspathe  von  Pantelleria.     Z.  X.  1883.  VIII.  125. 


THE  PLAGIOGLASES.  311 

The  series  of  triclinic  potash-soda  feldspars,  one  of  whose  most 
important  properties  must  be  that  they  possess  an  apparent  cleavage 
angle  P l\M,  which  varies  scarcely  any  from  a  right  angle,  and  yet  must 
do  so,  is  to  be  designated  as  the  series  of  anorthoclases  in  distinction  to 
the  plagioclases  which  plainly  cleave  obliquely. 

The  anorthoclases  are  isomorphous  mixtures  of  Ab  and  Or  in  the 
ratio  of  2  : 1  to  4.5  : 1 ;  that  is,  Ab^Oi^  to  Ab4.5Orn  to  which  is  added 
An  in  varying  amount.  The  ratio  An  :  Ab  -f-  Or  varies  from  1 :  3  to 
1 : 22.  The  habit  is  like  that  of  the  other  feldspars,  but  there  is 
occasionally  a  type  in  which  the  crystals  are  developed  in  prisms  par- 
allel to  c.  T and  /  predominate;  M sinks  to  almost  nothing.  Of  the 
macrodomes,  y  is  the  only  one  which  occurs.  The  triclinic  character 
is  very  obscure.  Twinning  according  to  the  Carlsbad,  Baveno,  and 
Manebacher  law  is  very  common;  the  separate  individuals  (halves  of 
the  twin)  are  multiple  twins  after  the  albite  and  pericline  law.  The 
twin  lamellae  are  almost  always  of  the  most  extreme  fineness,  so  that 
P  and  M  are  apparently  plane  faces,  and  appear  to  intersect  at  right 
angles.  A  third  law  of  lamellar  arrangement  occurs  locally :  twinning 
axis  the  normal  to  y  (201). 

The  lamellae  twinned  according  to  the  pericline  law,  that  is,  parallel 
to  the  rhombic  section,  are  inclined  4°-6°,  rarely  8°,  to  the  cleavage 
parallel  to  P  on  the  M  face  in  the  negative  sense ;  hence  in  the  oppo- 
site direction  to  those  of  albite,  to  which  anorthoclase  stands  nearest 
chemically.  The  cleavage  is  parallel  to  P  and  JtT,  as  in  all  feld- 
spars. The  specific  gravity  lies  between  that  of  orthoclase  and  albite, 
2.57-2.60,  and  rises  with  the  percentage  of  albite.  It  is  exactly  the 
same  as  for  the  perthites. 

Index  of  refraction  low  ;  fina  =  1.504-1.581,  according  to  Forstner, 
not  determined  directly,  but  calculated  from  the  axial  angle.  Double 
refraction  somewhat  stronger  than  for  orthoclase.  The  extinction  on 
P  is  positive,  and  varies  between  5°  45'  and  1°  30';  it  is  also  positive 
on  M)  and  lies  between  6°  and  9°  48',  so  that,  if  the  percentage  of 
anorthite  be  overlooked,  it  appears  to  grow  less  on  P  with  the  albite 
percentage,  and  to  increase  on  M.  The  twin  lamination  is  often  only 
visible  on  basal  sections  when  they  are  the  thinnest  possible,  because 
of  the  minuteness  of  the  lamellae  ;  anorthoclase  is  best  distinguished 
from  orthoclase  in  sections  at  right  angles  to  P  and  M.  In  these  sec- 
tions, also,  highly  twinned  areas  pass  into  others  free  from  twinning 
without  there  being  any  visible  boundary  between  them.  The  positive 
bisectrix  emerges  from  M^  as  in  all  feldspars,  and  with  slight  inclina- 
tion ;  it  bisects  the  obtuse  axial  angle ;  the  negative  acute  bisectrix 


312        PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

emerges  approximately  normal  to  y.  About  this  the  dispersion  is 
distinctly  horizontal,  and  p  >  v.  2Ena  varies  from  71°  40'  to  88°  27'. 

Anorthoclases  are  known  to  occur  in  the  augite  syenites  of  Southern 
Norway,  and  perhaps  in  their  porphyritic  equivalents,  as  well  as  in 
siliceous  varieties  of  the  amphibole  and  augite  andesites.  From  a 
consideration  of  the  rock  analyses,  it  is  probable  that  they  will  be 
found  in  trachytes  and  rhyolites,  as  well  as  in  dacites. 

2.  Appendix. — Fouque*  described  a  very  remarkable  feldspar 
from  Quatro  Ribeiras,  on  Terceira,  one  of  the  Azores.  It  has  the 
composition  of  a  CaO-  and  K3O-bearing  albite,  but  possesses  optical 
properties  which  approach  those  of  microcline  very  closely  ;  it  has  fine 
lamellar  twinning,  and  specific  gravity  —  2.593.  The  extinction  angle 
on  P  is  1°  30',  on  M it  is  +  9°  to  +  9°  30'.  Almost  normal  to  M 
stands  a  positive  bisectrix,  which  bisects  the  obtuse  axial  angle.  About 
the  negative  bisectrix,  which  is  almost  normal  to  y,  the  dispersion  is 
distinctly  horizontal,  and  p  >  v .  ZE '=  65°  40'  to  75°.  The  indices  of 
refraction  are  <xna  =  1.5234,  fina  =  1.5294,  yna  =  1.5305.  Changes  of 
temperature  up  to  200°  C.  are  without  effect  on  it. 


Disthene. 
Literature. 

M.  BAUER,  Beitrage  zur  Kenntniss  der  krystallographischen  Verhaltnisse  des 
Cyanits.  Z.  D.  G.  G.  1878.  XXX.  283-326 ;  1879.  XXXI.  244-254.  1880. 
XXXII.  717-728. 

F.  BECKE,    Die  Gneissformation  des  niederosterreichischen  Waldviertels      T  M 

P.  M.  1882.  IY.  225-231. 

E.  COHEN,  Ueber  einen  Eklogit,  welcher  als  Einschluss  in  den  Diamantgruben  von 
Jagersfontein,  Orange-Freistaat,  Slid  Afrika,  vorkommt.  N.  J.  B  1879  864- 
870. 

G.  VOM  RATH,  Ein  Beitrag  zur  Kenntniss  der  Krystallisation  des  Cyanit.     Z  X 

1879.  III.  1-12  ;  1881.  V.  17-23. 

E.  R.  RIES,  Untersuchungen  ilber  die  Zusammensetzung  des  Eklogits     T  M  P  JVI 
1878.  I.    195-198. 

Disthene  occurs  in  rocks  as  crystals  or  as  columnar  crystalloids, 
and  also  in  parallel  columnar  aggregates,  less  frequently  in  twisted 
ones.  The  crystals  are  only  well  crystallized  in  the  prism  zone, 
and  are  elongated  parallel  to  the  prism  axis;  terminal  faces  are  not 

*  Feldspath  triclinique  de  Quatro  Ribeiras  (He  de  Terceira).     Bull.  Soc.  min. 


DISTHENE.  313 

so  rare,  but  are  usually  so  uneven  and  bent  that  they  furnish  no 
measurable  angles.  Hence  sections  parallel  to  the  prism  zone  are 
lath-shaped,  with  round,  jagged,  or  quite  irregular  ends  ;  at  right  angles 
to  this  zone  they  are  six-sided,  with  one  large  and  two  small  edges,  Gl- 
are rounded.  By  the  suppression  of  one  pair  of  faces  the  basal  sections 
become  obliquely  rhombic.  The  predominant  faces  are  M=<x>Pao 
(100),  T  =  ooPco  (010),  I  =  ooP/  (110),  o  =  oo/P  (110),  P=oP 
(001),  k=  ooP/  2  (210).  The  most  important  angles  are  M/\T  = 
106°  4',  M AZ  =  145°  13',  M/\o  =  131°  42',  Pf\M=  101°  30',  PA  T 
=  105°  4',  according  to  G.  vom  Rath's  calculation.  Twinning  is  very 
frequent,  and  takes  place  after  the  following  laws:  (1)  Twinning  axis 
normal  to  M.  The  faces  P  and  T  form  protruding  and  re-entrant 
angles ;  this  is  the  most  common  law,  and  is  often  repeated  poly  syn- 
thetically. (2)  Twinning  axis  normal  to  the  edge  M :  T7  lying  in  the 
face  M,  composition  plane  M.  The  faces  T  form  re-entrant  angles. 
(3)  Twinning  axis  the  edge  M :  T,  composition  plane  M.  The  faces 
P  form  re-entrant  angles.  (4)  Twinning  axis  normal  to  P,  generally 
repeated  a  number  of  times,  and,  as  Bauer  has  shown,  it  is  a  pressure 
twinning.  Crossed  twins  like  staurolite,  twinned  parallel  to  a  face  (212), 
are  not  uncommon  in  the  smaller  crystals  of  paragon ite  schists;  their 
vertical  axes  intersect  at  about  60°. 

The  cleavage  parallel  to  Jtf  is  very  perfect,  and  gives  rise  to  sharp 
cracks,  which,  however,  do  not  traverse  the  whole  section  when  in 
rather  thick  plates.  The  cleavage  parallel  to  T  is  less  distinctly  notice- 
able microscopically  ;  its  cracks  are  shorter  and  rougher,  end  abruptly, 
and  are  less  numerous.  The  parting  parallel  to  P  corresponds  to  a 
gliding  plane,  as  Bauer  has  shown.  Hence,  longitudinal  sections  ex- 
hibit more  or  less  sharp  cracks  parallel  to  the  length  of  the  section,  and 
fissure-like  cracks  at  right  angles  to  it;  cross-sections  show  distinct 
cleavage  cracks  parallel  to  the  longest  edge,  sometimes  with  another 
set  parallel  to  one  of  the  shorter  edges. 

Oyanite  becomes  transparent  and  colorless ;  many  varieties,  how- 
ever, are  blue  or  greenish  blue.  The  pigment  is  generally  dissemi- 
nated quite  irregularly.  The  crystals  may  become  almost  opaque  from 
carbonaceous  matter.  The  index  of  refraction  is  high  (Des  Cloizeanx 
determined  fip  =  1.720),  therefore  the  surface  is  quite  rough,  the  mar- 
ginal total  reflection  strong.  The  double  refraction  must  be  not  in- 
considerable because  of  the  height  of  the  interference  colors,  which  is 
greater  than  that  of  andalusite,  but  less  than  that  of  sillimanite.  The 
axial  plane  stands  almost  normal  to  M]  and  its  trace  cuts  this  face  like 
the  diagonal  of  the  acute  plane  angle  on  M  from  the  edge  P :  M9  with  an 


314 


PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 


/ 

5                P      S 

K 

/ 

\ 

/ 

T, 

...   Q 

\ 

a/ 

v~ 

o 

M 

•% 

T 

\ 

/ 

inclination  of  about  30°  to  the  edge  M:  T  (Fig.  121).  The  character 
of  the  double  refraction  is  negative  ;  the  dispersion  about  a  weak,  v<p. 
The  axial  angle  is  generally  quite  large,  so  that  the  axes  often  are  only 
visible  on  M  in  oil ;  2  J^  —  82°-83°  ;  but  smaller  axial  angles  occasion- 
ally occur  (Litchfield).  The  extinction  on  Jffrom  the  edge  M:  T  is 
30°-31°,  on  ^from  the  same  edge  7°-8°;  on  P  it  is  approximately  par- 
y  j  S^  allel  and  normal  to  the  cleavage  parallel  to  J/,  and 

in  convergent  light  an  inclined  positive  bisectrix 
emerges  from  P.  The  trace  of  the  axial  plane 
runs  obliquely  to  the  cleavage.  Twins,  according 
to  the  1st  law,  are  not  recognizable  in  polarized 
light,  since  the  axial  plane  is  the  same  in  both 
individuals.  Those  following  the  other  laws  in 
which  M  is  the  composition  plane  give  extinc- 
tions symmetrical  to  the  twinning  boundary,, 
whose  difference  may  reach  60°.  The  twinning 
boundary  lies  parallel  to  the  cleavage  parallel  to 
Fig.  ijsi  jyr  jn  sections  of  the  zone  M :  T  and  M:P,  but 

intersects  these  cleavage  cracks  at  considerable  angles  in  oblique  sec- 
tions. 

Pleochroism  is  only  noticeable  in  distinctly  colored  varieties ;  the- 
colors  vary  between  bluish  and  colorless,  and  are  strongest  in  sections 
parallel  to  T. 

Disthene  is  generally  free  from  inclusions  :  inclusions  of  biotite  plate& 
or  those  of  colorless  mica,  and  of  specular  iron,  quartz  grains,  tourma- 
line and  rutile  needles,  are  rare ;  fluid  inclusions  more  so.  Their 
arrangement  is  without  order ;  but  the  mica  plates  usually  lie  along 
the  principal  cleavage,  M. 

Specific  gravity,  3.5-3.7.  Unattacked  by  acids.  Hence  it  is  easily 
isolated  from  the  rocks.  Chemical  composition  and  reactions  like  those 
of  andalusite.  Proper  decomposition  is  very  seldom  observed ;  a  finely 
laminated  aggregate  of  mica  appears  to  be  developed  from  disthene, 
which  is  then  clouded  by  limonite. 

Disthene  is  highly  characteristic  of  crystalline  schists,  gneisses, 
granulites,  paragonite  schist,  muscovite  schist,  and  eclogite,  and  ap- 
pears to  occur  especially  where  these  rocks  are  of  metamorpbic  origin. 
It  is  almost  universally  accompanied  by  garnet. 


COSSYRITE.  315 

Axinite. 

When  axinite  occurs  in  rocks  it  usually  forms  irregular  grains, 
which  are  rarely  bounded  crystallographically  by  the  hatchet-like 
forms.  The  cleavages  which  truncate  the  sharp  edges  of  the  faces 
P  (110)  and  u  (110),  and  P  (110)  and  r  (111),  are  but  imperfectly  ex- 
pressed  by  cracks  in  cross-sections. 

By  transmitted  light  it  is  colorless  to  very  light  yellowish,  pale 
grayish  brown,  or  violet.  Index  of  refraction  high,  and  double  re- 
fraction strong.  Des  Cloizeaux  determined  <xp  =  1.6720,  ftp  =  1.6779, 
yp  =  1.6810,  av  =  1.6850,  ftv  =  1.6918,  yv  =  1.6954.  The  bisectrix  is- 
normal  to  x  (111).  The  axial  angle  is  large,  so  that  the  axes  do  not 
emerge  from  sections  at  right  angles  to  the  bisectrix  in  air.  2  Vp  = 
71°  38'  to  74°  17',  2  Vv  =  71°  49'  to  74°  39'.  Character  of  the  double- 
refraction  negative ;  inclined  and  horizontal  dispersion  very  distinct. 

The  pleochroism,  d—  pale  olive-green  to  colorless,  b  =  dark  violet- 
blue,  c  —  cinnamon-brown,  is  scarcely  noticeable  in  thin  sections. 

Specific  gravity  =  3.3.  Chemical  composition  not  known  exactly, 
a  boron-bearing  lime-alumina  silicate.  Not  attacked  by  acids. 

Bears  no  interpositions  besides  the  associated  minerals,  chiefly 
tremolite  and  chlorite,  and  occasional  fluid  inclusions. 

Axinite  occasionally  occurs  on  the  borders  of  diabases  and  granites,, 
and  among  their  contact  products.  Zirkel  *  described  a  mixture  of 
axinite,  light-greenish  augite,  dark-green  hornblende,  quartz,  calcite, 
titanite  and  iron-ores,  called  limurite,  from  the  valley  of  Lesponne,  in 
the  Pyrenees. 

Cossyrite. 

Literature. 

H.  F6RSTNER,  Ueber  Cossyrit,  ein  Mineral  aus  den  Liparitlaven  von  Pantelleria. 
Z.  X.  1881.  V.  348-362. 

The  crystal  forms  of  cossyrite,  which  has  not  yet  been  completely 
in  vestigated,  are  very  similar  to  those  of  hornblende,  and  exhibit  almost 
monoclinic  symmetry.  Crystals  only  1.5  mm.  long  and  0.5  mm.  broad 
show  the  prism  and  both  vertical  pinacoids  in  the  prism  zone.  The 
prism  exhibits  the  most  noticeable  difference  of  angle  as  compared 
with  hornblende ;  oo/P:  ooP/  —  134°  09'.  It  is  placed  in  the  triclinic 
system  chiefly  from  the  fact  that  the  crystals  are  almost  always  twins 
parallel  to  ooPc»  (010). 

Cossyrite  cleaves  very  readily  parallel  to  both  prism  faces. 

*  Limurite  aus  der  Vallee  de  Lesponne.     N.  J.  B.  1879.     379. 


316          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

It  only  becomes  transparent  occasionally  in  very  thin  section^ 
Microlites  of  cossyrite  appear  coffee-brown  to  rust-brown  by  transmitted 
light.  Extinction  angle  on  ccPco  (100)  from  the  cleavage  parallel  to 
(110)  =  3°,  on  the  longitudinal  face  (010)  from  the  same  cleavage, 
39°.  The  rays  most  inclined  to  the  prism  axis  appear  to  be  the  most 
strongly  absorbed. 

Sp.  gr.  =  3.74-3.75.  Chemical  composition  approaching  that  of  a 
hornblende  rich  in  iron  and  soda.  Fuses  readily  to  a  brownish-black 
glass,  and  is  strongly  attacked  by  boiling  hydrochloric  acid.  It  forms 
a  constituent  of  the  acid  dacitic  lavas  of  the  island  of  Pantelleria. 


SERPENTINE.  317 


HOMOGENEOUS  AGGEEGATES. 

THE  doubly  refracting  aggregates,  when  of  sufficiently  fine  grain, 
are  characterized  under  the  microscope  by  the  fact  that  their  thin  sec- 
tions do  not  become  dark  in  any  position  between  crossed  nicols.  For 
the  different  substances  composing  the  aggregate  lie  beside,  through, or 
over  one  another,  in  such  a  way  that  their  principal  optical  sections 
never  coincide.  Consequently  the  phenomena  produced  are  those  de- 
scribed on  page  88.  The  distribution  of  the  colors  in  aggregates  be- 
tween crossed  nicols  generally  indicates  the  structure  of  the  aggregates, 
a  granular,  fibrous,  or  scaly  aggregate  structure  corresponding  to  a 
speckled,  striped,  or  flaked  change  of  colors. 

Serpentine. 

Literature. 

R.  VONDRASCHE,  Ueber  Serpentin  und  serpentinahnliche  Gesteine.  T.  M.  M.  1871.  1. 

E.  HUSSAK,  Ueber  einige  alpine  Serpentine.     T.  M.  P.  M.  1882.  V.  61-81. 
G.  TSCHERMAK,  Ueber  Serpentinbildung.     S.  W.  A.  1867.  July  No.  LVI. 

M.  WEBSKY,  Ueber  die  Krystallstructur  des  Serpentins  und  einiger  demselben  zuzu- 

rechnenden  Fossilien.     Z.  D.  G.  G.  1856.  X.  277. 
B.  WEIGAND,  Die  Serpentine  der  Vogesen.     T.  M.  M.  1875.  183-206. 

F.  J.  WIIK,  Mineralogiska  och  petrografiska  meddelanden.     Finska  Vet.  Soc.  F5r- 

handl.     Helsingfors.  1875. 

Serpentine  has  a  fibrous  or  apparently  laminated  structure  according 
to  the  parent  mineral  from  which  it  originated ;  still  the  apparent 
scales  may  represent  bundles  of  parallel  fibres.  The  arrangement  of 
the  fibres  varies  greatly,  sometimes  parallel,  at  other  times  confusedly 
felty,  the  optical  phenomena  between  crossed  nicols  changing  with 
the  arrangements  and  dimensions  of  the  fibres.  In  the  parallel  aggre- 
gates, which  are  not  too  finely  fibrous,  it  is  evident  that  they  are  bi- 
axial with  very  large  axial  angle,  whose  negative  bisectrix  is  normal  to 
the  axis  of  the  fibres,  which  is  also  the  axis  of  least  elasticity.  These 
fibres  have  a  low  index  of  refraction  (very  nearly  the  same  as  that  of 
Canada  balsam),  and  not  inconsiderable  double  refraction.  Chrysotile 
shows  these  relations  very  distinctly.  In  the  fine,  confusedly  fibrous 
aggregates  there  may  exist  such  a  perfect  compensation  that  they  often 
appear  isotropic. 


318          PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

The  mineral  from  which  serpentine  is  most  frequently  derived  is 
olivine.  On  page  216  this  alteration  is  described  in  its  incipiency,  as 
well  as  the  development  of  a  peculiar  net-like  structure.  When  ser- 
pentine is  typically  developed  three  forms  may  be  quite  distinctly 
made  out.  First,  dark  veins  and  bands  (PI.  XX VI.  Fig.  4)  of  a  deep- 
colored  leek-green  and  blue-green  serpentine  substance,  which  is  often 
opaque  from  metallic  oxides.  These  bands,  generally  keeping  the  same 
direction  for  some  distance,  evidently  correspond  to  the  first  cracks  and 
cleavage  in  the  olivine,  from  which  the  whole  process  started.  They 
sometimes  have  a  laminated  structure,  each  layer  being  fibrous  cross- 
wise. Within  the  meshes  of  the  large  net  a  smaller  net  is  found  to 
some  extent,  made  up  of  grass-green  serpentine  veins  generally  crooked 
and  intersecting,  which  by  higher  powers,  especially  in  polarized  light, 
exhibit  a  fibrous  structure  at  right  angles  to  the  length  of  the  veins ; 
these  bands  do  not  carry  metallic  oxides.  These  are  also  absent  from 
the  yellowish  green  serpentine  substance,  which  fills  the  meshes  of  the 
small  nets  and  evidently  corresponds  to  the  olivine  grains  which  were 
metamorphosed  last.  Here  the  structure  is  very  finely  scaly  and  scaly 
fibrous. 

In  the  alteration  of  hornblende  and  actinolite  into  serpentine  the 
cleavage  and  transverse  parting  of  amphibole  is  clearly  brought  out  by 
the  arrangement  of  the  serpentine  fibres.  Parallel  fibrous  aggregates 
of  serpentine  stand  normal  to  the  cleavage  of  the  hornblende,  while  the 
spaces  within  these  cleavage  cracks  are  confusedly  fibrous.  Between 
crossed  nicols  the  parallel  fibrous  lines,  which  partly  run  parallel  to  one 
another,  partly  intersect  at  124°-! 25°,  or  make  rhombs  with  other 
angles  and  also  rectangles,  stand  out  brightly  colored  from  the  dark 
ground  of  the  confusedly  fibrous  spaces.  This  gives  rise  to  a  highly 
characteristic  structure  which  Weigand  has  called  grating  or  "  win- 
dow"-structure  (PL  XXVI.  Fig.  5). 

Other  serpentines  which  have  a  laminated  structure  macroscopically, 
consist  of  microscopically  laminated  masses  crossing  at  right  angles  and 
exhibiting  a  knitted  structure  (PL  XXVI.  Fig.  4).  Scales  which  can 
be  loosened  give  in  convergent  light  the  interference  figure  of  a  biaxial 
mineral  with  small  axial  angle  about  a  negative  bisectrix  normal  to  the 
face  of  the  plate.  Thus  they  behave  very  nearly  or  exactly  the  same 
as  many  bastites.  These  serpentines  appear  to  have  been  produced 
from  monoclinic  pyroxenes,  as  Hussak  and  others  have  shown. 

The  fibrous  serpentines  or  chrysotile  serpentines,  which  arise  from 
olivine  and  hornblende,  could  be  placed  optically  parallel  with  the 
micaceous  serpentines,  bastite  or  antigorite  serpentines,  by  assuming 


DELESSITE.  319 

that  each  leaf  of  the  latter  is  built  up  of  parallel  fibres,  whose  longer 
axis  lies  parallel  to  the  vertical  axis  of  the  pyroxene.  But  the  axial 
angle  for  the  chrysotile  serpentines  is  considerably  greater  than  for  the 
antigorite  serpentines.  It  appears  to  grow  so  large  that  the  axis  of 
smallest  elasticity,  lying  along  the  axis  of  the  fibre,  becomes  the  acute 
bisectrix,  as  in  metaxite,  from  Schwarzenberg  in  Saxony,  according  to 
Websky's  observation. 

All  serpentines  are  transparent  and  greenish,  bluish  green  or  yel- 
lowish brown,  often  nearly  colorless,  seldom,  rust-red.  They  frequently 
-contain  the  interpositions  belonging  to  the  parent  mineral,  as  well  as 
unaltered  remnants  of  the  latter.  They  are  often  permeated  with  opal 
and  with  carbonates. 

The  different  varieties  of  serpentine,  as  metaxite,  picrolite,  marmo- 
lite,  retinalite,  jenkinsite,  vorhauserite.  picrosmine,  schwartzerite,  etc., 
have  been  described  by  Websky  (1.  c.),  Wiik  (1.  c.),  "Des  Cloizeaux, 
v.  Drasche  (T.  M.  M.  1871.  57),  and  Fischer  (Kritische  mikroskopisch- 
mineralogische  Studien.  1.  Fortsetzung.  Freiburg  i.  B.  1871.  31.  47). 

Sp.  gr.  —  2.5-2.7.  Chemical  composition  =  2H2O,  3MgO,  2SiO2. 
It  is  attacked  quite  vigorously  by  hydrochloric  acid,  especially  at  high 
temperatures,  with  the  separation  of  gelatinous  silica.  Sulphuric  acid 
acts  more  energetically  than  hydrochloric. 

Delessite. 

Delessite  forms  aggregates  mostly  with  divergent  fibrous  structure, 
which  may  take  the  shape  of  very  perfect  spherulites.  When  it  fills 
arnygdaloidal  cavities  it  is  in  layers  or  bands  parallel  to  the  rock 
boundary,  each  layer  consisting  of  fibres  standing  normal  to  the  rock 
walls.  The  different  layers  correspond  to  as  many  interruptions  in  its 
growth.  Delessite  becomes  transparent  and  green  or  yellowish  brown. 
Index  of  refraction  and  double  refraction  small.  Extinction  apparently 
parallel  and  normal  to  the  axis  of  the  fibres,  which  is  the  axis  of  least 
elasticity.  Pleochroism  of  varying  intensity ;  rays  vibrating  parallel 
to  the  axis  of  the  fibre  are  greenish ;  those  at  right  angles  yellowish, 
greenish  white  to  nearly  colorless. 

Sp.  gr.  —  2.5-2.6.  Chemical  composition  not  known  exactly — a 
hydrous  aluminous  silicate  of  iron  and  magnesia.  Easily  decomposed 
by  acids  with  the  separation  of  gelatinous  silica ;  when  heated  to  red- 
ness it  becomes  opaque  brownish  black  to  black. 

Delessite  forms  pseudomorphs  after  pyroxene  and  amphibole,  or  it 
fills  amygdaloidal  cavities  in  basic  rocks  in  combination  with  carbonates 


320         PHYSIOGRAPHY  OF  THE  ROCK-MAKING  MINERALS. 

and  epidote.     Grengesite  is  identical  with  delessite  both  in  structure 
and  physical  behavior. 

Kaolin. 

Literature. 

A.  KNOP,  Beitrage  zur  Kentniss  der  Steinkohlenformation  und  des  Rothliegenden 

im  Erzgebirgischen  Bassin.     N.  J.  B.  1859.  593-594. 
E.  E.  SCHMID,  Die  Kaoline  des  thuringischen  Buntsandsteins.     Z.  D.  G.  G.  1876, 

XXVIII.  87-111. 

Kaolin  forms  loose  earthy  aggregates,  which  are  produced  by  the 
weathering  of  feldspar,  elseolite,  scapolite,  and  other  minerals.  Iso- 
lated and  loosened  in  water,  these  aggregates  are  found  to  consist  of 
extremely  fine,  irregularly  bounded  plates,  which  are  rarely  hexagonal, 
and  are  completely  colorless.  In  the  large-leaved  varieties,  known  as 
nakrite  or  pholerite,  rhombic  or  hexagonal  forms  have  been  recognized, 
which  would  correspond  to  a  combination  of  a  prism  of  120°  with  a 
brachypinacoid  and  a  basal  plane.  The  plates  in  polarized  light  prove 
to  be  partly  crossed  trillings  parallel  to  a  prism  face.  Fibrous  struc- 
ture which  has  been  mentioned  by  some  observers  is  probably  brought 
about  by  a  rosette-like  arrangement  of  the  plates. 

Loose  scales  of  kaolin  are  transparent  and  colorless.  Th6  index  of 
refraction  is  about  the  same  as  that  of  Canada  balsam ;  the  double 
refraction  is  strong.  A  negative  bisectrix  emerges  from  the  face  of 
the  plate,  the  axial  plane  bisects  the  acute  prism  angle.  The  optical 
behavior  is  therefore  very  similar  to  that  of  muscovite.  Aggregates 
of  kaolin  are  cloudy  and  scarcely  translucent. 

Sp.  gr.  =  2.2-2.65.  Chemical  composition  =  2H2O,  A12O3,  2SiOa. 
Is  not  acted  on  by  hydrochloric  acid.  Is  decomposed  by  boiling  sul- 
phuric acid.  It  can  only  be  distinguished  with  certainty  from  color- 
less mica  by  chemical  reaction,  by  proving  the  absence  of  alkali ;  its 
specific  gravity  cannot  be  used  to  advantage  because  of  the  micaceous 
form  of  both  minerals. 


INDEX. 


Acmite,  242 
Actinolite,  249 
Acute  bisectrix,  38 
^Egirine,  242 

Aggregate  polarization,  88 
Aggregates,  20,  88,  115,  317 
"  homogeneous,  317 

spherical,  89 
Albite,  292,  306 
Albite  twinning,  290,  294 
Allanite,  272 
Almadine,  130 

Aluminium:  chemical  reactions  for,  113 
Amorphous  substances,  115,  121 
Amphiboles:  monoclinic,  244 

"  orthorhombic,  210 

Analcite,  140 
Analyzer,  47 
Anatase,  149 
Andalusite,  193 
Andesine,  292,  308 
Anhydrite,  192 
Anisotropic  media:  optical  properties  of, 

23 
Anisotropic  media:  double  refraction  in, 

31 

Anisotropic  minerals,  115 
Anomalies:  optical,  119 
Anomite,  261 
Anorthite,  292,  309 
Anorthoclase,  311 
Anthophyllite,  212 
Apatite,  177 
Apparatus  for  separating  mineral  grains, 

102,  106 
Aragonite,  192 
Arfvedsonite,  252 
Arrangement  of  inclusions,  18 
Augite,  240 
Automolite,  128 
Axes  of  elasticity,  35 

their  relative  value  in 

doubly  refracting  plates,  64 
Axes:  optic,  39,  41 
Axis:  optic,  33,  36 
Axinite,  315 
Axiolite,  90 

Bastite,  209 
Baveno  twinning,  278 
Bertrand's  ocular,  64 
Biaxial  crystals,  36 


Biaxial  crystals,  behavior  in  convergent 

polarized  light,  72,  74 
Biaxial  crystals:  indices  of  refraction  in, 

37 

Biaxial  crystals:  pleochroism  of,  86 
Biotite,  259 
Bisectrices,  38 
Braun's  heavy  solution,  101 
Breunnerite,  177 
Bro'gger's  apparatus,  102 
Bronzite,  202,  204 
Brookite,  190 
Brucite,  167 
Bytownite,  292,  309 

Calcite,  174 

Calcium:  chemical  reactions  for,  112 

Canada  balsam,  2 

Cancrinite,  181 

Carbonaceous  matter,  123 

detection  of,  97 

Carbonates:  chemical  detection  of,  93 
Carlsbad  twinning,  277 
Cassiterite,  151 
Cement  for  rock  sections,  2 
Chalcedony,  172 

Chemical  deformation  of  crystals,  12 
Chemical  detection  of  aluminous  miner- 
als, 98 

Chemical  detection  of  carbonaceous  sub- 
stances, 97 
Chemical  detection  of  carbonates,  93 

"  "          "  gelatinizing     sili- 

cates, 95 

Chemical  detection  of  native  iron,  95 
"        "  phosphates,  94 
"  "         "  protoxide  of  iron, 

Chemical  detection  of  sulphides,  94     [97 
Chemical  investigation  of  thin  sections, 

93 
Chemical  properties  of  the  rock-making 

minerals,  91 

Chemical  reactions,  110  v  [110 

"  "  Boricky's    method, 

Chemical  reactions  for  aluminium,  113 
calcium,  112 
chlorine,  113 
iron,  113 
magnesium,  112 
phosphorus,  114 
potassium,  111 
sodium,  112 


322 


INDEX. 


Chemical  reactions  for  sulphur,  114 
"   titanium,  114 

Chiastolite,  195 

Chlorine:  chemical  reactions  for,  113 

Chlorite,  186 

Chlorite  group,  185 

Chlorite  spar,  266 

Chloritoid,  266 

Chroinite,  126 

Classification  of  minerals  optically,  115 

Cleavage,  21 

Clinochlor,  186 

Color  of  minerals,  83 

Color  scale  of  Newton,  58 

Conical  refraction,  37 

Convergent  polarized  light:  investigation 
in,  67 

Convergent  polarized  light:  biaxial  plates 
perpendicular  to  an  optic  axis  in,  72 

Convergent  polarized  light:  the  same  per- 
pendicular to  a  bisectrix  in,  74 

Convergent  polarized  light:  interference 
phenomena  in,  68 

Cordierite,  217 

Corroded  crystals,  12 

Corundum,  166 

Cossyrite,  315 

Couzeranite,  157 

Critical  angle,  26 

Crossed  dispersion  of  the  optic  axes,  80 

Crystallites,  9 

Crystallization:  abnormal, 8 
normal,  6 

Crystal  sections,  4 

Crystals  in  fluid  inclusions,  17        % 

Cumulites,  9 

Cyan  He,  313 

Damourite,  265 

Deformation  of  crystals:  chemically,  12 
"  "  mechanically,  11 

Delessite,  319 

Diaclasite,  208 

Diallage,  238 

Dipyre,  157 

Direction  of  extinction  in  doubly  ~ef ract- 
ing  plates,  63 

Dispersion  of  light,  25 

Dispersion  of  the  optic  axes,  39  [80 

Dispersion  of  the  optic  axes:  crossed,  41, 

Dispersion  of  the  optic  axes  :  horizontal, 
41,  79 

Dispersion  of  the  optic  axest  inclined, 
41,  78 

Dispersion  of  the  optic  axes:  in  the  mono- 
clinic  system,  78 

Dispersion  of  the  optic  axes:  in  the  or- 
thorhombic  system,  76 

Dispersion  of  the  optic  axes:  in  the  tri- 
clinic  system,  80 

Disthene,  312 

Dolomite,  176 

Double  refraction  in  anisotropic  media,  31 


Double  refraction  in  crystals  with  a  prin- 
cipal axis,  32 

Double  refraction  in  crystals  without  a 
principal  axis,  34 

Doubly  refracting  media,  32 

"  "        plates  in  parallel  polar- 

ized light,  53 

Doubly  refracting  plates  at  right  angles 
to  an  optic  axis  in  polarized  light,  60 

Doubly  refracting  plates:  several  upon 
one  another  in  polarized  light,  61 

Dumortierite,  225 

Elaeolite,  179 
Elasticity:  axes  of,  34 

ellipsoid  of,  32 
Electro-magnet,  106 
Ellipsoid  of  elasticity,  32 
Enstatite,  202,  204 
Epidote,  269 
Etched  figures,  96 
Eucolite,  185 
Eucryptite,  181 
Eudialyte,  185 
Extinction  angle,  226 
Extinction  in  doubly  refracting  plates, 


Extraordinary  ray,  33 

Fassaite,  241 

Feldspars:  rnonoclinic,  276 
triclinic,  288,  292 
Figures:  etched,  96 

"        interference;  of  biaxial  plates, 
74,  75 
Figures:  interference:  of  uniaxial  plates, 

68 
Figures:  percussion,  256 

"       pressure,  257 
Fluid  inclusions,  15 

"  crystals  in,  17 

"  moving  bubbles  in,  17 

Fluorite,  128 
Forms  of  growth,  8 
Funnel  for  separating  mineral  grains,  103 

Gahnite.  128 
Garnet,  130    I 
Garnet  group,  129 
Gas  inclusions,  14 

"  "          secondary,  15 

Gedrite,  212 

Gelatiuization  of  thin  sections,  93 
Gelatinizing  silicates:  chemical  detection 

of,  95 

Glass  inclusions,  18 
Glasses,  121 
Glaucophane,  252 
Gliding  planes,  22 
Globospherites.  10 
Globulites,  9 

Grains  of  minerals:    method  of  mount- 
ing for  study,  3 


INDEX. 


323 


Grains  of  minerals:  mechanical  investi- 
gation of,  98 

Granophyre  structure,  19 
Granospherites,  90 
Graphite,  162 
Grating  structure,  318 
Grinding  of  thin  sections,  2 
Grossular,  130 
Gypsum,  227 

•Halos,  86 

Harada's  separating  apparatus,  102 
Hardness  of  isolated  grains  determined, 

110 

Haiiyne,  137 

Heating  thin  sections  to  red  heat,  97 
Heating  of  minerals  irregularly,  optical 

effects  of,  45 

Heavy  solutions:  Braun's,  101 
Klein's,  100 
Rohrbach's,  101 
Thoulet's,  99 
Hematite,  162 

Hercynite,  128  [161 

Hexagonal  system:  characteristics  of ,  115, 
Hobb's  specific  gravity  indicators,  104 
Homogeneous  aggregates,  317 
Horizontal  dispersion,  79 
Hornblende,  250 
Hour-glass  forms,  232 
Hyaline  substances,  121 
Hyalite,  122 
Hydrofluoric  acid:  separation  by  means 

of,  108 
Hypersthene,  202,  206 

Illumination  of  mineral  sections,  27 
Ilmenite,  164 
Inclined  dispersion,  78 
Inclusions,  14 

fluid,  15 
gas,  14 
glass,  18 

individualized,  18 
slag,  18 
Index  of  refraction,  23,  28 
Index  of  refraction  in  doubly  refracting 

plates,  66 
Index   of  refraction   tabulated   for   the 

rock-making  minerals,  66 
Indices  of  refraction  in  biaxial  crvstals, 

37 

Individualized  inclusions,  18 
Influence  of  temperature  and   pressure 

on  double  refraction,  42 
Interference  colors  of  doubly  refracting 

plates,  59 

Interference  colors:  order  of,  60 
"  cross  and  rings,  71 

figure  of  biaxial  plates,  74 
"  uniaxial       "      68 
phenomena  in    convergent 
light,  68 


Intergrowth  of  minerals,  19 
Iron:  chemical  reactions  for,  113 

"      native:  chemical  detection  of ,  95 
Isometric  system:  characteristics  of,  115, 

124 
Isotropic  media,  23 

"        refraction  in,  23 
minerals,  115  [52 

plates  in  parallel  polarized  light, 
Ittnerite,  140 

Jade,  250 
Jadeite,  243 

Kaolin,  320 

Kelyphite,  132 

Klein's  heavy  solution,  100 

Labradorite,  292,  308 
Lateral  pressure,  45 
Laws  of  refraction,  24 
Lepidolite,  264 
Leucite,  132t 
Leucoxene,  165 
Limiting  angle,  26 
Lithionite,  263 
Longulites,  9 

Magnesite,  177 

Magnesium:  chemical  reactions  for,  112 

Magnet,  electro,  106 

Magnetic  pyrites,  162 

Magnetite,  124 

Malacolite,  237 

Manebacher  twinning,  278 

Margarites,  9 

Masonite,  266 

Measurements  under  the  microscope,  5 

Mechanical  deformation  of  crystals,  11 

separation  of  mineral  grains, 
Meionite,  156  [98 

Melilite,  159 
Mica  group,  254 
Mica  plate,  for  determining  the  relative 

values  of  axes  of  elasticity,  64 
Mica  plate,  for  determining  the  optical 

character  of  minerals,  81 
Microchemical  investigation  of  mineral 

grains,  98 
Microcline,  288 
Microlites,  11 
Microlitic  structures,  10 
Micrometer,  5 

Micropegmatitic  structure,  19 
Microperthite,  286 
Microscope:  description  of,  50 
Microtine  habit,  297 
Mimetic  forms,  44 
Monoclinic  amphiboles,  244 
feldspars,  276 
pyroxenes,  230 
"  system:  characteristics  of,  40, 

115,  226 


324 


INDEX. 


Monoclinic    system:    dispersion    of    the 

optic  axes  in,  78 
Morphological  characters,  4 
Moving  bubbles  in  fluid  inclusions,  17 
Muscovite,  264 

Native  iron:  chemical  detection  of,  95 

Natrolite,  224 

Negative  crystals,  34 

Nepheline,  179 

Nephrite,  250 

Newton's  color  scale,  58 

Nicol  prism,  48 

Normal    symmetrical    position    of    the 

optic  axes,  41 
Nosean,  137 

Obtuse  bisectrix,  38 

•Oiigoclase,  292,  307 

Olivine,  212 

Omphacite,  241 

Opal,  121 

Optic  axis,  33,  36,  60 

Optic  axes:  dispersion  of,  39 

normal,   symmetrical    posi- 
tion of,  41 
Optic  axes:  symmetrical  position  of,  41 

"     section:  principal  one,  33,  39 
Optical  properties  of  minerals,  23 

"      character    of  doubly  refracting 

plates,  80 
Optical  characteristics  of  the  isometric 

system,  115,  124 
Optical  characteristics  of  the  tetragonal 

system,  115,  144 
Optical  characteristics  of  the  hexagonal 

system,  115,  161 
Optical  characteristics  of  the  orthorhom- 

bic  system,  40,  115,  189 
Optical  characteristics  of  the  monoclinic 

system,  40,  115,  226 
Optical  characteristics  of    the    triclinic 

system,  42,  115,  287 
Optical  classification  of  minerals,  115 
Optically  anomalous  minerals,  119 
Order  of  interference  colors,  60 

' '       determined 

by  the  quartz  wedge,  65 
Ordinary  ray,  33 
Orthoclase,  277 

Orthorhombic  amphiboles,  210 
pyroxenes,  200 
system :  characteristics  of, 

40,  115,  189 
Ottrelite  group,  266 

Paragonite,  265 

Parallel  polarized  light:  doubly  refract- 
ing plates  in,  53 

Parallel  polarized  light:  isotropic  plates 
in,  52 

Parallel  polarized  light:  investigation  in, 


Pennine,  186 

Percussion  figure,  256 

Pericline  twinning,  294 

Perofskite,  141 

Phenomena  of  strain,  44 

Phlogopite,  263 

Phosphates:  chemical  detection  of,  94 

Phosphorus:  chemical  reactions  for,  114 

Physical  properties,  21 

Picotite,  128 

Pilite,  217 

Plagioclase  group,  292 

Pleochroic  halos,  86 

"     in  cordierite,  219 
Pleochroism  of  biaxial  minerals,  86 
"  uniaxial        "        85 
Pleochroism  produced  artificially,  97 
Pleonaste,  127 
Poicolitic  structure,  19 
Polarization:  aggregate,  88 

of  light,  30 

Polarized  light:  convergent,  67 
Polarized    light:      convergent:     biaxial 

plates  perpendicular  to  an  axis  in,  72 
Polarized  light:  convergent:  biaxial  plates 

perpendicular  to  a  bisectrix  in,  74 
Polarized  light:  convergent:  interference 

phenomena  in,  68 
Polarized    light:     parallel:     anisotropic 

trimmed  crystals  in,  62 
Polarized  light:  parallel:  several  doubly 

refracting  plates  upon  one  another  in, 

61 
Polarized  light:  parallel:  isotropic  plates 

in,  52 

Polarized  light:  parallel:  doubly  refract- 
ing plates  in,  53 

Polarized  light:  parallel:  doubly  refract- 
ing plates  perpendicular  to  an   optic 

axis  in,  60 
Polarized  light:    parallel:  investigation 

in,  46 

Polarizer,  47 

Polarizing  instruments,  46 
microscope,  50 
Porodine  substances,  121 
Positive  crystals,  34 

"        determined  by  the  mica 

plate,  81 
Positive    crystals    determined    by    the 

quartz  wedge,  82 

Potassium:  chemical  reactions  for,  111 
Preparation  of  thin  sections,  2 
Pressure  figure,  257 
Pressure  planes,  22 
Principal  indices  of  refraction,  37 

"        optic  section,  33 
Prism:  nicol,  48 
Protoxide  of   iron:   chemical   detection 

of,  97 

Pseudobrookite,  191 
Pyrite,  124 
Pyrope,  131 


825 


Pyroxenes:  inonoclinic,  230 

orthorhombic,  200 
Pyrrhotite,  162 

Quarter  undulation  mica  plate,  81 

Quartz,  168 

Quartz  wedge,'65 

"  "  for  determining  the  op- 
tical character  of  doubly  refracting 
minerals,  82 

Quartz  wedge  for  determining  the  order 
of  interference  colors,  65 

Ray:  extraordinary,  33 

"     ordinary,  33 
Reflection:  total,  26 
Refraction  in  isotropic  media,  23 

index  of,  23,  28 
"  "      "    in  doubly  refracting 

plates,  66 

Refraction:  conical,  37  [37 

"          indices  of:  in  biaxial  crystals, 

Refraction:  double:  in  anistropic  media, 

37 
Refraction    double:    in  crystals  with  a 

principal  axis,  32 
Refraction:  double:  in  crystals  without  a 

principal  axis,  34 
Refraction:  double:  influenced  by  lateral 

pressure  and  irregular  heating,  45 
Relief  in  mineral  sections,  26 
Ripidolite,  186 

Rohrbach's  heavy  solution,  101 
Rough  surface  of  mineral  sections,  26 
Rubellan,  262 
Rutile,  145 

Sagenite  web,  146 

Sanidine,  277 

Scapolite  group,  154 

Schillerization  infcypersthene,  206 

Scolecite,  225 

Secondary  gas  cavities,  15 

Sections  of  minerals,  4 
"        "  rocks,  2 

Separation  of  minerals  by  chemical 
means,  108 

Separation  of  minerals  by  mechanical 
means,  according  to  specific  gravity, 
99 

Separation  of  minerals  by  the  electro- 
magnet, 106 

Separating  apparatus,  102 
funnel,  103 
sieves,  98 

Shelly  structure,  13 

Sericite,  265 

Serpentine,  317 

.Serpentinization  of  olivine,  216 

Sieves  for  separating  rock  powder,  98 

Sillimanite,  195 

Sismondine,  266 

Skeleton  crystals,  10 


Skolopsite,  140 

Slag  inclusions,  18 

Sodalite,  136 

Sodalite  group,  135 

Sodium:  chemical  reactions  for,  112 

Solution  planes,  207 

Solutions  for  the  mechanical  separation 

of  minerals,  99 
Specific  gravity  balance,  105 

"        indicators,  104 
"  "       of  isolated  powder  de- 

termined, 109 

Specific  gravity  of  .the  rock-making  min- 
erals tabulated,  110 
Specific  gravity  used  for  the  separation 

of  minerals,  99 
Specular  iron,  162 
Spessartine,  131 
Spherical  aggregates,  89 
Spherulites,  10 
Spinel  group,  127 
Stauroscopic  methods  for    determining 

the  direction  of  extinction,  63 
Staurolite,  198 
Strain  phenomena,  44 
Structure:  granophyre,  19 
grating,  318 
hour-glass,  232 
microlitic,  10 
micropegmatitic,  19 
poicolitic,  19 
shelly,  13 
zonal,  13 

Sulphur:  chemical  reactions  for,  114 
Sunstoue,  308 

Symmetrical  position  of  the  optic  axes,  41 

System  of  crystallization:  isometric,  124 

"  tetragonal,  144 

hexagonal,  161 

"  "  orthorhombic, 

189 

System  of  crystallization:  monoclinic,  226 
triclinic,  287 

Table  of  the  indices  of  refraction  of  the 
rock-making  minerals,  66 

Table  of  the  optical  characteristics  of  the 
feldspars,  300 

Table  of  the  relation  between  the  nega- 
tive axial  angle  and  iron  percentage  in 
the  orthorhombic  pyroxenes,  203 

Table  of  the  variations  in  the  axial  angle 
and  the  extinction  angle  in  the  mono- 
clinic  pyroxenes,  235 

Table  of  the  variations  in  the  optical 
characteristics  of  the  monoclinic  am- 
phiboles,  246 

Table  of  the  specific  gravities  of  the  rock- 
making  minerals,  110 

Table  of  the  specific  gravities  of  the  min- 
eral indicators,  104 

Table  of  the  specific  gravities  of  the 
feldspars,  305 


326 


Table  of  the  theoretical  composition  of 
the  plagioclase  feldspars,  292 

Table  of  Newton's  color  scale,  58 

Talc,  223 

Tetragonal  system:  characteristics  of ,  115, 
144 

Thin  sections:  use  of,  1 

"  preparation  of,  2 

Thoulet's  heavy  solution,  99 

Thulite,  222 

Titanite,  274 

Titanium:  chemical  reactions  for,  114 

Topaz,  197 

Total  reflection,  26 

Tourmaline,  182 

"  tongs,  47 

Tremolite,  249 

Trichites,  10 

Tridymite,  173 

Triclinic  system:  characteristics  of,  42, 
115,  287 


Twins,  19 

Twinning  laws:  albite,  294 

"      Baveno,  278 

"      Carlsbad,  277 
"  "      Manebacher,  278 

"      pericline,  294 

Uniaxial  crystals,  34  [68 

"  "          interference  figure  of, 

Uralite,  253 
Uralitization,  241 

Yesuvianite,  157 

Westphal's  balance,  105 
Wollastonite,  228 

Zinnwaldite,  263 
Zircon,  152 
Zoisite,  221 
Zonal  structure,  13 


ITION 


EXPLANATION   OF  PLATES. 


PLATE  I. 
FIG.  1.  Solution  of  sulphur  in  a  mixture  of  carbon  bisulphide  and  Canada  balsam. 

Globulites  and  crystals  have  formed  with  a  halo  free  from  globulites. 

X200. 
FIG.  2.  The  same.     About  the  larger  globulites  have  been  formed  halos  free  from 

globulites;  about  these  as  well  as  about  gas-bubbles  diffusion-streams  may 

be  recognized.     X  200. 

FIG.  3.  The  same.     Globulites  and  longulites  have  formed:     X  200. 
FIG.  4.  Globulites  in  basalt-glass  from  Hawaii,  Sandwich  Islands.     X  250. 
FIG.  5.  Longulites  and  cumulites  of  sulphur  in  a  solution  of  sulphur  in  a  mixture  of 

carbon  bisulphide  and  Canada  balsam.     X  200. 
FIG.  6.  Globospherite  and  globulite  in  the  same  solution.     X  200. 

PLATE  II. 

FIG.  1.  Margarites  in  obsidian.     Clear  Lake,  Cal.     X  250. 
FIG.  2.  Trichites  in  obsidian.     Mexico.     X  200. 

FIG.  3.  Microlites  and  trichites  in  schillerizing  obsidian.     Transcaucasus.     X  225. 
FIG.  4.  Spherulites  (sphcerocrystalle)  in  obsidian.     Lipari.     X  15. 
FIG.  5.  Cumulite  in  felsite  pitchstone.   Buschbad  in  Triebischthal,  Saxony.    X  450. 
FIG.  6.  Augite  with  colorless  crystallization-halo  in  trachytic  pitchstone.     Hammers- 
fjord,  Iceland.     X  25. 

PLATE  III. 

FIG.  1.  Skeleton  crystal  of  olivine.    Palma.     X  100. 

FIG.  2.  Skeleton  crystal  of  magnetite  in  glassy  basalt.     Schatung,  China.     X  90. 

FIG.  3.  Skeleton  crystal  of  augite  in  pitchstone.    Arran.     X  90. 

FIG.  4.  Spherical  aggregates  and  bundles  of  feldspar  crystals  in  trachyte.     Caucasus. 

X60. 

FIG.  5.  Broken  feldspar  crystals  in  augite  andesite.     Grad-Jakan,  Java.     X  33. 
FIG.  6.  Bent  and  frayed-out  mica  in  augite  minette.    Fuchmuhle  near  Weinheim  on 

the  Bergstrasse.     X  42.  i 

PLATE  IV. 

FIG.  1.  Shattered  garnet  in  mica  schist.     Brixen,  Tyrol.     X  10. 
FIG.  2.  Marginally  fractured    feldspar  in    anorthite    rock.     Chicontrini,    Quebec, 

Canada.     X  10. 
FIG.  3.  Albite,  fractured  and  dislocated.     In  ordinary  light.     From  phillite-gneiss. 

Allen's  Creek,  Victoria,  Australia.     X  10. 
FIG.  4.  The  same  between  crossed  nicols. 
FIG.  5.  Crystal  of  biotite,  corroded  and  with  pressure  figures,  from  porphyrita.     II- 

feld,  Hartz.     X  22. 
FIG.  6.  Plagioclase  with  bent  lamellae,  between  crossed  nicols;  from  olivine  gabbro. 

Store  Bekkaf  jord,  Norway.     X  45. 


328  EXPLANA  2  INDE£  PL  A  TES. 

PLATE  V. 
FIG.  1.  Corroded  quartz  crystal  from  quartz  porphyry  from  Scharfenstein,  Miinster- 

thal,  Black  Forest,     x  25. 
FIG.  2.  Corroded  nosean  crystal  from  leucitophyre  from  Burgberg  near  Rieden. 

X12. 

FIG.  3.  Zonal  structure  in  plagioclase  in  melaphyre.     Bufaure,  Fassathal.     X  36. 
FIG.  4.  Zonal  structure  in  melanite  in  phonolite.     Steinriesenweg  near  Oberbergen, 

Kaiserstuhl.     X  60. 
FIG.  5.  Zonal  structure  in  augite  in  leucitite.     Kreuzle  near  Rothweil,  Kaiserstuhl. 

X21. 
FIG.  6.  Hour  glass-like  zonal  structure  in  augite  in  nephelinite.      Eichberg  near 

Rothweil,  Kaiserstuhl.     X  45. 

PLATE  VI. 

FIG.  1,  Gas  inclusions  in  obsidian.     Mexico.     X  80. 
FIG.  2.  Fluid  inclusions  in  apatite.     Pfitsch,  Tyrol.     X  150. 
FIG.  3.  Fluid  inclusions  in  rock  salt.     Friedrichshall,  Wurtemherg.     X  200. 
FIG.  4.  Two  fluids,  which  do  not  mix,  in  one  inclusion  in  smoky  quartz.     Branch- 

ville,  Conn.     X  100. 
FIG.  5.  Fluid  inclusions  with  separated  crystal  in  quartz  from  granite  porphyry- 

Cornwall.     X  210. 
FIG.  6.  Fluid  inclusions,  which  do  not  wet  the  walls  of  the  cavity,   in   topaz. 

Schneckenstein,  Saxony.     X  84. 

PLATE  VII. 

FIG.  1.  Glass  inclusions  in  labradorite  from  Monte  Pilieri,  Etna.     X  50. 
FIG.  2.  Glass  inclusions  in  quartz,  dihexahedral,  from  quartz  porphyry  from  Dos- 

senheim  on  the  Bergstrasse.     X  72. 

FIG.  3.  Glass  inclusions  with  several  bubbles  in  oligoclase.     Pantelleria.     X  100. , 
FIG.  4.  Quartz  inclusions  in  heulandite.     Faroe.     X  30. 
FIG.  5.  Microlite  inclusions  in  hypersthene.     St.  Paul's  Island.     X  30. 
FIG.  6.  Central  accumulation  of  inclusions  in  feldspar  from  trachyte  from  Monte 

Olebano  near  Pozzuoli,  Naples.     X  5. 

PLATE  VIII. 
FIG.  1.  Peripheral  accumulation  of  inclusions  in  feldspar  from  hornblende  andesite. 

South  Siberia.     X  54. 
FIG.  2.  Zonal  arrangement  of  inclusions  in  leucite   from  Vesuvian  lava.     Monte 

Somma.     X  13. 
FIG.  3.  Interpenetratiou  of  quartz  and  ortho6lase  in  granophyre.     Sperberbachel  near 

Hohwald,  Vosges.     X  80. 
FIG.  4.  Aggregate  of  quartz  grains  in  ordinary  light,  in  granite-porphyry.     Gross- 

sachsener  Thai,  Odenwald.     X  24. 
FIG.  5.  The  same  between  crossed  nicols.    The  figure  should  be  turned  about  90°  to 

the  left. 
FIG.  6.  Penetration  of  augite  by  plagioclase,  magnetite,  and  augite  from  basalt. 

Lowenburg,  Siebengebirge.     X  30. 

PLATE  IX. 

FIG.  1.  Spherulites  (sphcerocrystalle)  of  chalcedony  between  crossed  nicols,  from  dia- 
base-porphyrite.     Pfalz.     x  50. 


EXPLANATION  OF  PLATES.  329 

PIG.  2.  Grains  of  oolite  between  crossed  nicols.    From  a  coral  reef,  Bahama  Islands. 

X20. 
FIG.  3.  Granospherite  of  quartz  between  crossed  nicols  from  Eisenkiesel.     Stifts- 

buckel,  Heidelberg,     x  25.  '  . 

FIG.  4.  Bertrand's  interference  crosses  on  sphserosiderite  from  Steinheim,  Wetterau. 

X35. 

FIG.  5.  Octahedral  cleavage;  section  parallel  to  0  (111)  on  fluorite.   Markirch.   X  42. 
FIG.  6.  Prismatic  cleavage;  section  parallel  to  oP  (001)  on  scapolite.    Oedegarden  near 

Bamle,  Norway.     X  150. 

PLATE  X. 
FIG.  1.  Rhombohedral  cleavage;  section  parallel  7?  (1011);   calcite.     Auerbach  on 

the  Bergstrasse.     X  30. 
FIG.  2.  Cleavage  parallel  to  the  prism  and  two  vertical  pinacoids  in  sections  parallel 

tooP(OOl);  hypersthene.     St.  Paul's  Island.     X  24. 
FIG.  3.  Pyramidal  cleavage,  section  parallel  to  oP  (001) ;  in  anatase.     Oisans,  Dau- 

phine.     X  24. 
FIG.  4.  Prismatic  cleavage,  section  at  right  angles  to  c,  in  augite  from  nepheline 

tephrite.     Neunlinden,  Kaiserstuhl.     X  39. 
FIG.  5.  Prismatic  cleavage,  section  at  right  angles  to  c,  in  hornblende  from  dacite. 

Timokthal,  Servia.     X  36. 
FIG.  6.  Pinacoidal  cleavage  in  a  section  at  right  angles  to  it;  mica  from  gramtite. 

Grasstein  near  Mauls,  Tyrol.     X  30. 

PLATE  XI. 

FIG.  1.  Cleavage  parallel  to  oP (001)  and  oo  Poo  (010)  in  a  section  parallel  to  ooPcb  (100) 

in  orthoclase  from  augite  syenite.     Laurvig,  Norway.     X  27. 
FIG.  2.  Cleavage  parallel  to  oP  (001)  and  ooPcc   (100)  in  a  section  parallel  to  ccPob 

(01 0)  in  epidote  from  epidote  rock  from  Auerbach  on  the  Bergstrasse.  x  60. 
FIG.  3.  Cleavage  parallel  to   GO  P  (110)  in  a  section  at  right  angles  to  c  in  titanite 

from  syenite  from  Lohrbach,  Odenwald.     X  75. 
FIG.  4.  Crystals  of  sodium  fluosilicate.     X  72. 
FIG.  5.  Crystals  of  sodium  fluosilicate  and  amorphous  aluminium  fluosilicate  from 

sodalite.     Vesuvius.     X  27. 
FIG.  6.  The  same.     X  160,  X  100,  and  X  140. 

PLATE  XII. 

FIG.  1.  Crystals  of  potassium  fluosilicate  from  apophyllite.     Fassathal.     X  130. 
FIG.  2.  Crystals  of  potassium  fluosilicate  from  sanidine.     Wehr.     x  140. 
FIG.  3.  Crystals  of  lithium  fluosilicate  and  aluminium  fluosilicate  from  Zinnwaldite. 

XlOO. 

FIG.  4.  Crystals  of  calcium  fluosilicate.     X  45. 
FIG.  5.  Crystals  of  calcium  fluosilicate  from  apophyllite.     X  42. 
FIG.  6.  Crystals  of  magnesium  fluosilicate  from  biotite.     x  30. 


PLATE  XIII. 

FIG.  1.  Gypsum  crystals,     x  20. 
FIG.  2.  Crystals  of  caesium  alum,     x  20. 
FIG.  3.  Crystals  of  ammonium  magnesium  phosphate  (struvite).     X  10. 


330  EXPLANATION  OF  PLATES. 

FIG.  4.  The  same  from  very  dilute  solution.     X  30. 

FIG.  5.  Ammonium-molybdenum  phosphate  in  crystals.     X  140. 

FIG.  6.  Crystals  of  zirconia.     X  120. 

PLATE  XIV. 

FIG.  1.  Hornblende  with  magnetite  border  and  wreath  of  augite  and  magnetite, 

from  nepheline  tephrite  from  Gran  Canaria.     X  130. 

FIG.  2.  Double  refraction  in  garnet,  in  a  section  parallel  to  o>  0  (110).  Peru,  x  9. 
FIG.  3.  Interpositions  in  garnet,  arranged  along  the  axial  planes;  quartzite  from 

Libramont,  Luxemburg.     X  15  and  X  24. 

FIG.  4.  Garnet  with  kelyphite  rim;  olivine  rock.  Karlstetten,  Lower  Austria.  X  15. 
FIG.  5.  Leucite  with  zonal  alternation  of  different  inclusions;  Vesuvian  lava.  X  165. 
FIG.  6.  Leucite  in  striated  melilite,  from  leucitite  from  Capo  di  Bove,  near  Rome. 

X48. 

PLATE  XV. 

FIG.  1.  Leucite  surrounded  by  augite  in  the  form  of  a  wreath,  from  leucitophyre. 

Olbruck,  Upper  Brohlthal.     X  80. 

FIG.  2.  Perofskite  in  melilite  basalt.    Spitzberg  near  Wartenberg,  Bohemia.     X  170. 
FIG.  3.  Titanite  after  rutile  in  amphibole  gneiss.     Oetzthal,  Tyrol.     X  100. 
FIG.  4.  Rutile  from  clay  slate  from  Kautenbach  in  Luxemburg  (isolated).     X  240, 

And  in  clay  slate  from  Hahnenbach  near  Kirn  on  the  Nahe.     X  250. 
FIG.  5.  Zircon  crystals  out  of  granitite  from  Strehlen,  Silesia  (isolated).      X  160  and 

X140. 
FIG.  t>.  Melilite  with  peg-structure  in  nepheline  basalt  from  Oahu,  Sandwich  Islands. 

X66. 

PLATE  XVI. 

FIG.  1.  Ilmenite  altered  peripherally  into  titanite  (leucoxene).     Alpbachthal  near 

Brixlegg,  Tyrol.     X  24. 
FIG.  2.  Ilmenite   almost   completely  altered   into   leucoxene ;    ' '  augite-propylite.  '* 

Schemnitz.     x  18. 

FIG.  3.  Corundum  in  norite.     Wolfsgrube  near  Klausen,  Tyrol.     X  72. 
FIG.  4.  Tridymite  in  trachyte  from  Pomasqui,  N.  Quito,  Equador.     X  84. 
FIG.  5.  Calcite  with  twin  lamellae  parallel  to  —  |  between  crossed  nicols.     X  45. 
FIG.  6.  Basal  section  through  nepheline  in  leucitophyre.    Olbruck,  Upper  Brohlthah 

X190. 

PLATE   XVII. 

FIG.  1.  Vertical  section  through  nepheline  in  leucitophyre  from   Olbruck,  Upper 

Brohlthal.     X  190. 
FIG.  2.  Basal  and  vertical  sections  of  andalusite;   andalusite  hornstone.     Andlau, 

Vosges.     X  25. 
FIG.  3.  Chiastolite  in  sections  parallel  and  inclined  to  the  base;  chiastolite  schist. 

Pyrenees.     X  18. 

FIG.  4.  Sillimanite  in  quartz;  out  of  gneiss  from  Freiberg,  Erzgebirge.     X  25. 
FIG.  5.  Vertical  section  of  bronzite.     Kupferberg,  Silecia.     X  57. 
FIG.  6.  Section  parallel  to  (010)  through  a  parallel  intergrowth  of  enstatite  and  diall- 

age.     Groditzberg  near  Liegnitz,  Silesia      X  36. 


EXPLANATION  OF  PLATES.  331 

PLATE  XVIII. 
FIG.  1.  Enstatite  altered  into  bastite,  melaphyre.     Hohenstein  near  Ilfeld,  Hartz. 

X  45. 
FIG.  2.  Olivine  with  symmetrically  formed  and  arranged  glass  inclusions,  out  of 

glassy  basalt.     Mauna  Loa,  Sandwich  Islands.     X  190. 
FIG.  3.  Twin-growths  of  olivine  crystals  parallel  to  Poo  (Oil),  out  of  nepheline  basalt 

from  Randen,  Hegau.     X  75. 
FIG.  4.  Rough  surface  of  olivine  in  Canada  balsam,  basalt.     Steinschonau,  Bohemia. 

X  57. 
FIG.  5.  Olivine  with  inclusion  of  groundmass  in  the  form  of  its  hoat;  basalt.    Sieben- 

biirgen.     X  87. 
FIG.  6.  Olivine  in  advanced  serpentinization;  olivine  norite.     Obere  Baste,  Harz- 

burg.     X  27. 

PLATE  XIX. 

FIG.  1.  Olivine  (hyalosiderite)  with  broad,  marginal  secretion  of  iron  oxide,  limburg- 

ite.     Limburg  near  Sasbach,  Kaiserstuhl.     X  42. 
FIG.  2.  Olivine    altered    into    hornblende    (pilite);    kersantite.     Marbach,  'Lower 

Austria.     X  42. 
FIG.  3.  Penetration  trilling  of  cordierite,  between  crossed  nicols.    Asama  Yama, 

Japan.     X  140. 
FIG.  4.  Zoisite  in  longitudinal  and  cross  sections;   amphibolite  from  Zamborinho 

near  Macedo,  Portugal.     X  25. 
FIG.  5.  Augite  twinned  parallel  to    oo  Poo  (100)  between  crossed  nicols;  palatinite 

from  Martinstein  near  Kreuznach.     X  22. 
FIG.  6.  Augite  with  twin  lamellae  parallel  to  0P(001),  diabase  from  New  Haven, 

Conn.    X  96. 

PLATE  XX. 

FIG.  1.  Intergrowth  of  augite  crystals,  limburgite.  Limburg,  Sasbach;  Kaiser- 
stuhl. X  18. 

FIG.  2.  Form  of  growth  in  augite;  Vesuvian  lava,  Monte  Somma.     X  60. 

FIG.  3.  Forms  of  growth  in  augite;  felsite  pitchstone  from  Corriegills  on  Arran. 
X  96  and  X  130. 

FIG.  4.  Augite  with  corroded  centre;  nephelinite.  Herberg  near  Oberbergen, 
Kaiserstuhl.  X  33. 

FIG.  5.  Cleavage  of  augite  in  sections  parallel  to  c;  leucite  basalt.  Vormberg  near 
Ihringen,  Kaiserstuhl.  X  33. 

FIG.  6.  Cleavage  of  diallage  parallel  to  ooP(110)and  ooPob  (100)  in  a  section  at 
right  angles  to  c.  Olivine-gabbro.  Hausdorf,  Silesia.  X  30. 

PLATE  XXI. 

FIG.  1.  Parallel  intergrowth  of  augite  and  hornblende.     Picrite  from  Heim  near 

Oberdieten,  Nassau.     X  42. 
FIG.  2.  Alteration  of  augite  into  chlorite.     Proterobase  from  Stiebitz  near  Bautzen, 

Saxony.     X  30. 
FIG.  3.  Hornblende  twinned  parallel  to  ooPob  (100)  in  basal  section  between  crossed 

nicols.     Out  of  amphibole  granite  from  Pre  de  Fouchon  near  Gerardmer, 

Vosges.     X  80. 


332  EXPLANATION  OF  PLATES. 

FIG.  4.  Zonal  structure  in  hornblende  in  basal  section,  quartz  diorite.     Little  Falk 

Minnesota.     X  80. 

FIG.  5.  Uralite  in  basal  section,  uralite  porphyrite.     Miask      X  30. 
FIG.  6.  Uralite  in  vertical  section,  uralite  porphyrite.     Predazzo,  Tyrol.     X  9- 

PLATE  XXII. 

FIG.  1.  Biotite  with  rutile  needles.     Out  of  diorite-porphyrite  from  Lippenhof  near 

Triberg,  Black  Forest.     X  120. 

FIG.  2.  Biotite  altered  to  chlorite.     Bodethal,  Hartz.     X  27. 
FIG.  3.  Biotite  altered  to  chlorite  and  epidote,  granite-porphyry.     Etival,  Vosges. 

X  15. 
FIG.  4.  Ottrelite   in    basal   section  with  zonal  structure,  phyllite.     Harvey  Hills. 

Leeds,  Canada.     X  56. 
FIG.  5.  Twinning  in  ottrelite  in  vertical  section  between  crossed  nicols,  phyllite. 

Ottrez,  Belgium.     X  24. 
FIG.  6.  Zonal  structure  in  the  form  of  an  hour-gla«s  in  ottrelite  in  vertical  section. 

From  the  same  locality.     X  27. 

PLATE  XXIII. 

FIG.  1.  Twinned  titauite  between  crossed  nicols,  elaeolite  syenite.     Foya,  Portugal. 

X66. 
FIG.  2.  Carlsbad  twin  of  sanidine  in  clinodiagonal  section,  phonolite.     Wolf's  Rock, 

Land's  End,  England.    X  15. 
FIG  j3.  Baveno  twin  of  sanidine  in  leucitophyre  from  Engler  Kopf  near  Rieden;  in 

polarized  light.     X  52. 
FIG.  4.  Zonal  structure  in   orthoclase,   brought  out  by  weathering,  in   a   section 

parallel  to  ooP  o>  (010),  amphibole  granitite.     Val  d'Ajol,  Vosges.     X  8. 
FIG.  5.  Broken  sanidine  crystal  in  phonolite.    Oberbergen,  Kaiserstuhl.     X  24. 
FIG.  6.  Transverse  parting  in  sanidine,  phonolite.     Hohenkriihen,  Hegau.     X  21. 

PLATE  XXIV. 

FIG.  1.  Parallel  iutergrowth  of  sanidine  and  plagioclase  between  crossed  nicols, 

trachyte.     Mont  Dore,  Auvergne.     X  48. 

FIG.  2.  Zeolitization  of  sanidine  in  phonolite  from  Hohentwiel,  Hegau.     X  25. 
FIG.  3.  Interpenetration   of   orthoclase   and   plagioclase,  between   crossed    nicols ; 

augite  gneiss.     Seyberer  Berg,  Lower  Austria.     X  24. 
FIG.  4.  Microperthitic  intergrowth  of  orthoclase  and  albite  in  a  vertical  section, 

granitite.     Moslawina,  Croatia.     X  75. 

FIG.  5.  The  same  in  different  sections,  gneiss.    Chicontrini,  Quebec,  Canada.    X  40. 
FIG.  6.  Orthoclase    altered    to    muscovite.       Granite-porphyry.      Erdmannsdorf, 

Silesia,     x  33. 

PLATE   XXV. 

FIG.  1.  Microcline,  section  parallel  to  0P(001),  between  crossed  nicols.     Arendal. 

X  21. 
FIG.  2.  Parallel  intergrowth  of    microcline  and  albite,    section  parallel  to  ooPob 

(010),  between  crossed  nicols.     Unterflockenbach,  Odenwald.    X  12. 
FIG.  3.  Lath-shaped  plagioclase,  nepheline  basanite.    Palma,  Canary  Islands.   X  87. 


EXPLANATION  OF  PLATES.  333 

FIG.  4.  Jagged  outline  of  plagioclase,  between  crossed  nicols;  basalt.  Same  local- 
ity X  24. 

FIG.  5.  Zonal  structure  of  plagioclase  with  different  optical  orientation  in  the  sepa- 
rate zones,  between  crossed  nicols;  felsite-pitchstone.  Cunardo  near 
Lugano.  X  39. 

FIG.  6.  Twin  striation  of  plagioclase  according  to  the  albite  law,  between  crossed 
nicols;  diabase.  Biella,  Piedmont.  X  45. 

PLATE  XXVI. 

FIG.  1.  Twin  lamination  in  plagioclase  according  to  the  albite  and  pericline  law, 

between  crossed  nicols;  olivine  gabbro.     Le  Prese,  Veltlin.     x  12. 
FIG.  2.  Baveno  twin  of  plagioclase,  between  crossed  nicols.     Vesuvian  lava.     Torre 

dell'Annunziata.     1734.     X  45. 
FIG.  3.  Net-like  mtergrowth  of   plagioclase  with  glas  inclusions;    augite  andesite. 

Tokayer  Bahnhof ,  Hungary.     X  57. 
FIG.  4.  Serpentine  derived  from  olivine,  with  mesh  structure.     Schweidnitz,  Silesia. 

X24. 
FIG.  5.  Serpentine  derived  from  arnphibole,  with  grating  structure,  between  crossed 

nicols.     Rauenthal  near  Markirch,  Vosges.     X  45. 
FIG.  6.  Serpentine  derived  from  augite  with  bar  structure,  between  crossed  nicols. 

Sprechenstein  near  Sterzing,  Tyrol.     X  45. 


ERKATA. 

A 

Page    24,  third  line  from  top :  for  experienced  read  experiences. 
38,  tenth  line  from  top :  ft  should  be  C. 
69,  fifteenth  line  from  top  :  for  focus  read  locus. 
117,  twelfth  line  from  top  :  for  These  read  Those.  x 

125,  second  line  from  bottom :  for  'Imenite,  titanite  read  ilmenite  and  titanite. 
131,  third  line  from  top:  amphobolite  eclogite  should  be  amphibolite,  eclogite. — 
On  this  page,  the  reference-mark  before  the  second  footnote  should  be 
f  instead  of  %. 
171,  twenty-first  line  from  top .    there  should  be  a  comma  after  quartz  por- 


175,  twentieth  line  from  top:  for  CaO,CO3  read  CaO,CO2. 
179,  eleventh  line  from  bottom  :  for  rock  read  rocks. 

181,  footnotes  transposed  :   the  asterisk  (*)  should  be  prefixed  to  the  note 
beginning   "  J.   H.    Caswell,"  the  obelisk  (f)  to  the  note  beginning 
,/""-?•  "Elseolite  syenite,"  and  the  double-dagger  (ty  to  "Am.  Journ." 

/      s   /  240,  fifteenth  line  from  top :  for  diabasis  read  diabases. 

tenth  line  from  bottom:  there  should  be  a  comma  after  "vine"  at  the 

beginning  of  the  line. 

259,  twelfth  line  from  bottom  :  to     only  should  be  to  ft  only. 
264,  footnote:  for  L.  J.  read  N.  J.  B. 

285,  eighth  line  from  top  :  for  microscopically  read  macroscopically . 
290,  seventh  line  from  bottom  :  for  P :  M  read  P :  L 
324,  thirtieth  line  from  top  :  for  trimmed  read  twinned. 

ERRORS  IN  CRYSTALLOGRAPHIC  SYMBOLS. 

Page  il8,  sixth  line  from  top :  ooPcc  should  be  ooPob  . 

eleventh  line  from  top :  OoPco  "        "  GoPoo . 

119,  twenty- ninth  line  from  top:  OoPco  "         "  ooPoo  . 

173,  sixteenth  line  from  top:  doP  "        "  ooP. 

227,  eighth  line  from  bottom :  ooPo>  "        «  ooP<x>  . 
sixth  line  from  bottom :                    Poo  "        "  Poo  ;   and 

|Pob  «  ««  fPi. 

228,  eighth  line  from  bottom :              —  Pa)  «  "  —Poo. 
230,  second  line  from  bottom  :               iPob  "  "  JPoo  . 
242,  eighth  line  from  top :                     OoPoo  "  «  ooPob  . 
248,  fourth  line  from  top :                    ooPoo  «  -  ooPoo . 
252,  bottom  line :                                    ooPoo  «•  "  ooPoo  . 


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ROSENBUSCH,  PHYSIOGRAPHY.     Vol.  I. 


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